BACKPORT: Input: xpad - validate USB endpoint count during probe

This prevents a malicious USB device from causing an oops.

Change-Id: I47c27541a4c2f0cec354eb83b3013bb825ed6e90
Signed-off-by: Cameron Gutman <aicommander@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>