net: usb: asix: fill null-ptr-deref in asix_suspend

When asix_suspend() is called dev->driver_priv might not have been
assigned a value, so we need to check that it's not NULL.

Similar issue is present in asix_resume(), this patch fixes it as well.

Found by syzkaller.

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
Modules linked in:
CPU: 0 PID: 24 Comm: kworker/0:1 Not tainted 4.14.0-rc4-43422-geccacdd69a8c #400
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Workqueue: usb_hub_wq hub_event
task: ffff88006bb36300 task.stack: ffff88006bba8000
RIP: 0010:asix_suspend+0x76/0xc0 drivers/net/usb/asix_devices.c:629
RSP: 0018:ffff88006bbae718 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: ffff880061ba3b80 RCX: 1ffff1000c34d644
RDX: 0000000000000001 RSI: 0000000000000402 RDI: 0000000000000008
RBP: ffff88006bbae738 R08: 1ffff1000d775cad R09: 0000000000000000
R10: 0000000000000000...