Input: ad7879 - return plain error code from ad7879_probe()

With the switch to devm, there is no need for ad7879_probe() to return the
touchscreen structure, we can use plain error code. This also fixes issue
introduced with devm concersion, where we returned 0 on success (which
worked OK since IS_ERR(0) would not trigger, but was not correct
regardless).

Fixes: 381f688eee3d ("Input: ad7879 - use more devm interfaces")
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>