USB: lvstest: tighten endpoint sanity check

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: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>