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

sound/soc/codecs/cs-amp-lib.c

@@ -108,7 +108,7 @@ static efi_status_t cs_amp_get_efi_variable(efi_char16_t *name,
 
 	KUNIT_STATIC_STUB_REDIRECT(cs_amp_get_efi_variable, name, guid, size, buf);
 
-	if (IS_ENABLED(CONFIG_EFI))
+	if (efi_rt_services_supported(EFI_RT_SUPPORTED_GET_VARIABLE))
 		return efi.get_variable(name, guid, &attr, size, buf);
 
 	return EFI_NOT_FOUND;