[media] mceusb: buffer parsing fixups for 1st-gen device

If we pass in an offset, we shouldn't skip 2 bytes. And the first-gen
hardware generates a constant stream of interrupts, always with two
header bytes, and if there's been no IR, with nothing else. Bail from
ir processing without calling ir_handle_raw_event when we get such a
buffer delivered to us.

Signed-off-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>