Input: pxrc - fix freeing URB on device teardown

URB is the only resource that is not managed, and thus is destroyed too early,
before we unregister input device and stop URB in pxrc_close(). To fix it let's
install custom devm handler to free the URB at the right time in devm unwind
sequence.

Reviewed-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Tested-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>