staging:iio: make iio_sw_buffer_preenable much more general.

Also introduces active_scan_mask storage to tell the core what is
really being currently captured from the device (different from
what is desired as often has bonus channels).

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