ASoC: cs-amp-lib: Support Dell SSIDExV2 UEFI variable

ASoC: cs-amp-lib: Support Dell SSIDExV2 UEFI variable

Add a function cs_amp_devm_get_vendor_specific_variant_id() to return
a vendor-specific hardware identifier string (if there is one) and use
it to fetch an identifier from Dell SSIDExV2 UEFI variable content.

Dell use the same PCI SSID on multiple products that might have different
audio hardware and thus need different firmware for the amplifier DSP.
The SSIDExV2 string contains additional system identifiers, and the
second field is a 2-character audio hardware identifier.

There are older Dell models with Cirrus Logic amplifiers that have the
SSIDExV2 UEFI variable but do not have the 2-character audio ID in the
second field. The SSIDExV2 is ignored if the second field is not
2 characters.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20260121132243.1256019-2-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>