Input: xpad - remove spurious events of wireless xpad 360 controller

When powering up a wireless xbox 360 controller, some wrong joystick
events are generated. It is annoying because, for example, it makes
unwanted moves in Steam big picture mode's menu.

When my controller is powering up, this packet is received by the
driver:
00000000: 00 0f 00 f0 00 cc ff cf 8b e0 86 6a 68 f0 00 20  ...........jh..
00000010: 13 e3 20 1d 30 03 40 01 50 01 ff ff              .. .0.@.P...

According to xboxdrv userspace driver source code, this packet is only
dumping a serial id and should not be interpreted as joystick events.
This issue can be easily seen with jstest:
$ jstest --event /dev/input/js0

This patch only adds a way to filter out this "serial" packet and as a
result it removes the spurous events.

Signed-off-by: Clement Calmels <clement.calmels@free.fr>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>