staging: iio: ad9850.c: code cleanup

checkpath.pl was complaining about value_mask:
ERROR: Macros with complex values should be enclosed in parenthesis

I fixed this by simply removing it since it's not used (as well as another
macro).  Got rid of the un-necessary error_ret label as well.

Signed-off-by: Guillaume Morin <guillaume@morinfr.org>
Reported-by: Michael Welling <mwelling@ieee.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>