iio: ad9523: replace core mlock with local lock

This is also part of a long term effort to make the use of mlock opaque and
single purpose.

This lock is required for accessing device registers. The device may be
accessed by multiple processes at the same time, and this can result in
inconsistent data, where one device reads data before the other one has
finished writing.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>