staging:iio: Differential channel handling - use explicit flag rather than types.

Straight forward change in the core, but required some drivers to not use
the IIO_CHAN macro as that doesn't allow setting this bit (and is
going away anyway). Hence the churn.

Tested on max1363 with a couple of supported parts.

V2: differential bit in code got 7 bits and direction 1. Reversed that.
Issue spotted by Michael - thanks!

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>