ASoC: cs-amp-lib: Fix NULL pointer crash if efi.get_variable is NULL

Call efi_rt_services_supported() to check that efi.get_variable exists
before calling it.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Fixes: 1cad8725 ("ASoC: cs-amp-lib: Add helpers for factory calibration data")
Link: https://patch.msgid.link/20240805114222.15722-1-rf@opensource.cirrus.com


Signed-off-by: Mark Brown <broonie@kernel.org>