Input: elants_i2c - avoid divide by 0 errors on bad touchscreen data

The following crash may be seen if bad data is received from the
touchscreen.

[ 2189.425150] elants_i2c i2c-ELAN0001:00: unknown packet ff ff ff ff
[ 2189.430738] divide error: 0000 [#1] PREEMPT SMP
[ 2189.434679] gsmi: Log Shutdown Reason 0x03
[ 2189.434689] Modules linked in: ip6t_REJECT nf_reject_ipv6 rfcomm evdi
uinput uvcvideo cmac videobuf2_vmalloc videobuf2_memops snd_hda_codec_hdmi
i2c_dev videobuf2_core snd_soc_sst_cht_bsw_rt5645 snd_hda_intel
snd_intel_sst_acpi btusb btrtl btbcm btintel bluetooth snd_soc_sst_acpi
snd_hda_codec snd_intel_sst_core snd_hwdep snd_soc_sst_mfld_platform
snd_hda_core snd_soc_rt5645 memconsole_x86_legacy memconsole zram snd_soc_rl6231
fuse ip6table_filter iwlmvm iwlwifi iwl7000_mac80211 cfg80211 iio_trig_sysfs
joydev cros_ec_sensors cros_ec_sensors_core industrialio_triggered_buffer
kfifo_buf industrialio snd_seq_midi snd_seq_midi_event snd_rawmidi snd_seq
snd_seq_device ppp_async ppp_generic slhc tun
[ 2189.434866] CPU: 0 PID: 106 Comm: irq/184-ELAN000 Tainted: G        W
3.18.0-13101-g57e8190 #1
[ 2189.434883] Hardware name: GOOGLE Ultima, BIOS Google_Ultima.7287.131.43 07/20/2016
[ 2189.434898] task: ffff88017a0b6d80 ti: ffff88017a2bc000 task.ti: ffff88017a2bc000
[ 2189.434913] RIP: 0010:[<ffffffffbecc48d5>]  [<ffffffffbecc48d5>] elants_i2c_irq+0x190/0x200
[ 2189.434937] RSP: 0018:ffff88017a2bfd98  EFLAGS: 00010293
[ 2189.434948] RAX: 0000000000000000 RBX: ffff88017a967828 RCX: ffff88017a9678e8
[ 2189.434962] RDX: 0000000000000000 RSI: 0000000000000246 RDI: 0000000000000000
[ 2189.434975] RBP: ffff88017a2bfdd8 R08: 00000000000003e8 R09: 0000000000000000
[ 2189.434989] R10: 0000000000000000 R11: 000000000044a2bd R12: ffff88017a991800
[ 2189.435001] R13: ffffffffbe8a2a53 R14: ffff88017a0b6d80 R15: ffff88017a0b6d80
[ 2189.435011] FS:  0000000000000000(0000) GS:ffff88017fc00000(0000) knlGS:0000000000000000
[ 2189.435022] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 2189.435030] CR2: 00007f678d94b000 CR3: 000000003f41a000 CR4: 00000000001007f0
[ 2189.435039] Stack:
[ 2189.435044]  ffff88017a2bfda8 ffff88017a9678e8 646464647a2bfdd8 0000000006e09574
[ 2189.435060]  0000000000000000 ffff88017a088b80 ffff88017a921000 ffffffffbe8a2a53
[ 2189.435074]  ffff88017a2bfe08 ffffffffbe8a2a73 ffff88017a0b6d80 0000000006e09574
[ 2189.435089] Call Trace:
[ 2189.435101]  [<ffffffffbe8a2a53>] ? irq_thread_dtor+0xa9/0xa9
[ 2189.435112]  [<ffffffffbe8a2a73>] irq_thread_fn+0x20/0x40
[ 2189.435123]  [<ffffffffbe8a2be1>] irq_thread+0x14e/0x222
[ 2189.435135]  [<ffffffffbee8cbeb>] ? __schedule+0x3b3/0x57a
[ 2189.435145]  [<ffffffffbe8a29aa>] ? wake_threads_waitq+0x2d/0x2d
[ 2189.435156]  [<ffffffffbe8a2a93>] ? irq_thread_fn+0x40/0x40
[ 2189.435168]  [<ffffffffbe87c385>] kthread+0x10e/0x116
[ 2189.435178]  [<ffffffffbe87c277>] ? __kthread_parkme+0x67/0x67
[ 2189.435189]  [<ffffffffbee900ac>] ret_from_fork+0x7c/0xb0
[ 2189.435199]  [<ffffffffbe87c277>] ? __kthread_parkme+0x67/0x67
[ 2189.435208] Code: ff ff eb 73 0f b6 bb c1 00 00 00 83 ff 03 7e 13 49 8d 7c
24 20 ba 04 00 00 00 48 c7 c6 8a cd 21 bf eb 4d 0f b6 83 c2 00 00 00 99 <f7> ff
83 f8 37 75 15 48 6b f7 37 4c 8d a3 c4 00 00 00 4c 8d ac
[ 2189.435312] RIP  [<ffffffffbecc48d5>] elants_i2c_irq+0x190/0x200
[ 2189.435323]  RSP <ffff88017a2bfd98>
[ 2189.435350] ---[ end trace f4945345a75d96dd ]---
[ 2189.443841] Kernel panic - not syncing: Fatal exception
[ 2189.444307] Kernel Offset: 0x3d800000 from 0xffffffff81000000
	(relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[ 2189.444519] gsmi: Log Shutdown Reason 0x02

The problem was seen with a 3.18 based kernel, but there is no reason
to believe that the upstream code is safe.

Fixes: 66aee900 ("Input: add support for Elan eKTH I2C touchscreens")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>