spi: spi-nxp-fspi: Check return value of devm_mutex_init()

devm_mutex_init() can fail. With CONFIG_DEBUG_MUTEXES=y the mutex will
be marked as unusable and trigger errors on usage.

Add the missed check.

Fixes: 48900813 ("spi: spi-nxp-fspi: remove the goto in probe")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250617-must_check-devm_mutex_init-v7-1-d9e449f4d224@weissschuh.net