counter: ftm-quaddec: use devm_mutex_init()

counter: ftm-quaddec: use devm_mutex_init()

ftm_quaddec_probe() calls mutex_init() but neither the cleanup
action nor a remove callback issues a matching mutex_destroy(),
so the lock debug state is leaked on driver unbind.

Switch to devm_mutex_init() so the mutex is torn down in the same
devm scope it was set up in.

Signed-off-by: Stepan Ionichev <sozdayvek@gmail.com>
Reviewed-by: Joshua Crofts <joshua.crofts1@gmail.com>
Link: https://lore.kernel.org/r/20260523184351.7567-1-sozdayvek@gmail.com
Signed-off-by: William Breathitt Gray <wbg@kernel.org>