counter: intel-qep: Replace manual mutex logic with lock guards

Use guard() for handling mutex lock instead of locking and unlocking
mutex explicitly. This improves readability by eliminating the need for
gotos and by clearly indicating mutex will be locked only when
execution is in guard scope.

Signed-off-by: Joao Paulo Menezes Linaris <jplinaris@usp.br>
Co-developed-by: Guilherme Dias <guilhermeabreu200105@usp.br>
Signed-off-by: Guilherme Dias <guilhermeabreu200105@usp.br>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20260512173058.14858-1-jplinaris@usp.br
Signed-off-by: William Breathitt Gray <wbg@kernel.org>