iio: __iio_update_buffers: Slightly refactor scan mask memory management

Add a small helper function iio_free_scan_mask() that takes a mask and
frees its memory if the scan masks for the device are dynamically
allocated, otherwise does nothing. This means we don't have to open-code
the same check over and over again in __iio_update_buffers.

Also free compound_mask as soon a we are done using it. This constrains its
usage to a specific region of the function will make further refactoring
and splitting the function into smaller sub-parts more easier.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>