iio: imu: kmx61: Use guard(mutex)() over manual locking

iio: imu: kmx61: Use guard(mutex)() over manual locking

Include linux/cleanup.h to take advantage of new macros.

Replace manual mutex_lock() and mutex_unlock() calls across the file
with guard(mutex)() and scoped_guard() where appropriate to simplify
error paths and eliminate manual locking calls.

Add new helper function kmx61_read_for_each_active_channel() to mitigate
certain style issues and to prevent notifying that the IRQ is finished
whilst holding the lock.

Update certain returns, and add default case to return -EINVAL in
kmx61_read_raw().

Remove now-redundant gotos and ret variables, as the new RAII macros
make them unneeded.

Signed-off-by: Maxwell Doose <m32285159@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>