ALSA: hda/tas2781: handle missing EFI calibration data

The code does not properly check whether the calibration variable is
available in the EFI. If it is not available, it causes a NULL pointer
dereference.

Check the return value of the first get_variable call also.

BUG: kernel NULL pointer dereference, address: 0000000000000000
Call Trace:
 <TASK>
 ? __die+0x23/0x70
 ? page_fault_oops+0x171/0x4e0
 ? srso_alias_return_thunk+0x5/0x7f
 ? schedule+0x5e/0xd0
 ? exc_page_fault+0x7f/0x180
 ? asm_exc_page_fault+0x26/0x30
 ? crc32_body+0x2c/0x120
 ? tas2781_save_calibration+0xe4/0x220 [snd_hda_scodec_tas2781_i2c]
 tasdev_fw_ready+0x1af/0x280 [snd_hda_scodec_tas2781_i2c]
 request_firmware_work_func+0x59/0xa0

Fixes: 5be27f1e

 ("ALSA: hda/tas2781: Add tas2781 HDA driver")
CC: stable@vger.kernel.org
Signed-off-by: Gergo Koteles <soyer@irl.hu>
Link: https://lore.kernel.org/r/f1f6583bda918f78556f67d522ca7b3b91cebbd5.1702251102.git.soyer@irl.hu
Signed-off-by: Takashi Iwai <tiwai@suse.de>