Input: ad7879 - do not manipulate capability bits directly

Instead of manipulating capabilities bits of input device directly, let's
use input_set_capability() API.

Also, stop setting ABS_X/Y bits explicitly as input_set_abs_params() does
this for us.

Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>