staging:iio: Fix scan mask update

When updating the scan mask we have to check the actual scan mask for if the
channel is already enabled, not the matching scan mask from the available
scan masks. The bit will already be set there and as a result the actual
scan mask will not get updated and the channel stays disabled.

Also fix the return value of iio_scan_el_store which would return 1 instead of
the number of bytes written if the channel was already active in the scan mask.

Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>