platform/x86: xiaomi-wmi: Use devm_mutex_init()

platform/x86: xiaomi-wmi: 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
   3520	   1112	     64	   4696	   1258	drivers/platform/x86/xiaomi-wmi.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   3069	   1040	     64	   4173	   104d	drivers/platform/x86/xiaomi-wmi.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/bb5d7a57e11eb580f610276a351a01a993341fb8.1757239732.git.christophe.jaillet@wanadoo.fr
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>