Input: xpad - don't depend on endpoint order

The order of endpoints is well defined on official Xbox pads, but
we have found at least one 3rd-party pad that doesn't follow the
standard ("Titanfall 2 Xbox One controller" 0e6f:0165).

Fortunately, we get lucky with this specific pad because it uses
endpoint addresses that differ only by direction. We know that
there are other pads out where this is not true, so let's go
ahead and fix this.

Signed-off-by: Cameron Gutman <aicommander@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>