staging: iio: dac: Enable driver support for AD5444 and AD5446 DA converters

Enable support for AD5444 and AD5446:  12-/14-Bit High Bandwidth
Multiplying DACs with Serial Interface.

staging: iio: dac: Add support for AD5541A, AD5512A digital to analog convertors

staging: iio: dac: Fix according to review feedback

Review feedback by Jonathan Cameron:
	Remove spurious new line.
	Document struct members.
	Remove redundant variable initialization.

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