Input: ads7846 - use PTR_ERR_OR_ZERO()

Input: ads7846 - use PTR_ERR_OR_ZERO()

The PTR_ERR_OR_ZERO() helper function checks if a pointer contains an
errno code and returns it or return 0 if that's not the case. Use the
helper instead of open coding the same logic in the driver. This was
found with make coccicheck that complains with the following warning:

drivers/input/touchscreen/ads7846.c:532:1-3: WARNING: PTR_ERR_OR_ZERO can be used

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>