Input: ad7879 - make sure we set up drvdata

The conversion to devm accidentally removed setting up of I2C client data
upon successful probe of the touchscreen. Let's move this setting into the
core, so we do not forger about it again.

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>