counter: intel-qep: Use devm_mutex_init()

intel_qep_probe() calls mutex_init() but lacks the pairing
mutex_destroy() calls. Convert to devm_mutex_init() which handles
cleanup automatically.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Joshua Crofts <joshua.crofts1@gmail.com>
Reviewed-by: Stepan Ionichev <sozdayvek@gmail.com>
Link: https://lore.kernel.org/r/20260520111813.3934-1-ilpo.jarvinen@linux.intel.com
Signed-off-by: William Breathitt Gray <wbg@kernel.org>