ASoC: mediatek: mt8192: mt8192-afe-gpio: Use guard() for mutex locks

ASoC: mediatek: mt8192: mt8192-afe-gpio: Use guard() for mutex locks

Convert the explicit mutex_lock()/mutex_unlock() pair to guard(mutex)
to simplify the locking logic and automatically release the mutex on
all exit paths.

This changes the mutex release point from immediately before dev_warn()
to automatic cleanup at scope exit. However, the affected path only emits
a warning and immediately returns -EINVAL, without any further processing.

Signed-off-by: bui duc phuc <phucduc.bui@gmail.com>
Link: https://patch.msgid.link/20260610102021.83273-6-phucduc.bui@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>