gpio: pisosr: Use devm_mutex_init()

gpio: pisosr: Use devm_mutex_init()

Use devm_mutex_init() instead of hand-writing it.

This saves some LoC, improves readability and saves some space in the
generated .o file.

Before:
======
   text	   data	    bss	    dec	    hex	filename
   8431	   1808	    192	  10431	   28bf	drivers/gpio/gpio-pisosr.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   8112	   1736	    192	  10040	   2738	drivers/gpio/gpio-pisosr.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Andrew Davis <afd@ti.com>
Link: https://lore.kernel.org/r/01910ebdaba7d8d0cdc4ac60eb70da8e29cb85f1.1757251512.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>