Input: kxtj9 - remove unneeded retval variable

The retval variable isn't needed since isn't used in the function. Remove
the variable and just return 0 instead.

This also fixes the following make coccicheck warning:

drivers/input/misc/kxtj9.c:638:5-11: Unneeded variable: "retval". Return "0" on line 646

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