soundwire: honor clock_reg_supported in the clock scaling check

sdw_slave_set_frequency() treats class_id and prop.clock_reg_supported
as equivalent evidence that a slave implements the bus-clock base and
scale registers, but the bank-switch reprogramming path checks class_id
alone, so a class-0 slave that declared the registers never gets the
next-bank scale written there. The registers are SoundWire 1.2, not
SDCA, so a device may well implement them without setting the class
field.

Extend the helper to honor clock_reg_supported, as discussed with
Pierre-Louis in the WCD9378 review. This also makes a link whose
peripherals all declare clock_reg_supported eligible for dynamic clock
scaling in the generic bandwidth allocation, which is what declaring
the registers means.

With the helper extended, sdw_slave_set_frequency()'s open-coded test
computes the same predicate; call the helper there instead, so future
quirks or updates land in one place.

Link: https://lore.kernel.org/all/5717102b-f7ab-42b2-8065-064d94dd2bee@linux.dev/
Link: https://lore.kernel.org/all/6991398d-4ae4-45ee-85d0-3b66462fec1d@linux.dev/
Assisted-by: Claude:claude-fable-5
Signed-off-by: Jorijn van der Graaf <jorijnvdgraaf@catcrafts.net>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.dev>
Link: https://patch.msgid.link/20260728173542.61146-2-jorijnvdgraaf@catcrafts.net
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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