Use the new endpoint helpers to lookup the required interrupt-in
endpoint.

Note that this in fact both loosens and tightens the endpoint sanity
check by accepting any interface with an interrupt-in endpoint rather
than always using the first endpoint without verifying its type.
Signed-off-by: NJohan Hovold <johan@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>