HID: wacom: Delete unnecessary checks before the function call "input_free_device"

The input_free_device() function tests whether its argument is NULL and
then returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Jiri Kosina <jkosina@suse.com>