ALSA: hda/tas2781: Fix a potential race condition that causes a NULL pointer...

ALSA: hda/tas2781: Fix a potential race condition that causes a NULL pointer in case no efi.get_variable exsits

A a potential race condition reported by one of my customers that leads to
a NULL pointer dereference, where the call to efi.get_variable should be
guarded with efi_rt_services_supported() to ensure that function exists.

Fixes: 4fe23851 ("ALSA: hda/tas2781: Move and unified the calibrated-data getting function for SPI and I2C into the tas2781_hda lib")
Signed-off-by: Shenghao Ding <shenghao-ding@ti.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>