Input: ad7879 - pass up error codes from probe functions

If the sub-probe functions fail, we need to pass up the error code to the
higher levels from the probe function.  We currently always return 0 even
if there was an error so higher levels don't report problems.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>