Input: ad7879 - try parsing properties on non-DT systems

We have switched the driver to use generic device properties API, so there
is no need to check for presence of DT node before trying parse properties.

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