Input: xpad - enable force feedback on xbox 360 controllers only

Commit 4994cd8dadcf9d484ab3ec19f3c7c7a4e5353c1c introduced a
regression which causes xpad to report force feedback cababilities
for non-360 controllers too, even while there is no actual support
for those.

Fix that by adding a check for XTYPE_XBOX360 to xpad_init_ff().

Signed-off-by: Anssi Hannula <anssi.hannula@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>