staging:iio: Switch the channel masks to bitmaps so as to allow for more channels.

This is as light as possible on changes to current drivers.
Some drivers make assumptions that their masks fit in a single
long.  Given they were previously working this is clearly valid if
not tidy.

The max1363 is an example where there should be no such assumptions.

V2: Add the new ad5933

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <Michael.Hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>