Input: improve parsing OF parameters for touchscreens

When applying touchscreen parameters specified in device tree let's make
sure we keep whatever setup was done by the driver and not reset the
missing values to zero.

Reported-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>