ASoC: SDCA: Update text of FIXME

ASoC: SDCA: Update text of FIXME

A couple of attempts to correct this FIXME have been sent upstream but
the situation is not quite a simple as the FIXME implies. Update the
FIXME to include a better description of the situation.

Link: https://lore.kernel.org/linux-sound/20260408085607.3813488-1-shumingf@realtek.com/
Link: https://lore.kernel.org/linux-sound/20260324-sdca-function-status-init-irq-v1-1-bba49417a4e0@gmail.com/
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://patch.msgid.link/20260410104500.163337-1-ckeepax@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>