iio: adc: PAC1934: 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
  50985	  23992	    192	  75169	  125a1	drivers/iio/adc/pac1934.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  50654	  23920	    192	  74766	  1240e	drivers/iio/adc/pac1934.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/f92033415f43aa02fe862cb952e62b6ded949056.1757239464.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>