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

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

[ Upstream commit dc268085e499666b9f4f0fcb4c5a94e1c0b193b3 ]

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: 1cad8725f2b9 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>