iio: adc: ti-adc128s052: Use devm_mutex_init()

iio: adc: ti-adc128s052: Use devm_mutex_init()

Quoting Jonathan:
"Whilst it doesn't bring huge advantage, now we have devm_mutex_init()
it seems reasonable to use it and maybe catch a use after free for the
lock"

Switch to use devm_mutex_init() while working on this file.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://patch.msgid.link/efe4a039e2bfbf4dcf30743f6b7b88fce3b9ee39.1744022065.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>