Pinnacle class devices should return "88 07 xx" or "88 08 xx" when
entering command mode.  If either the first byte or the second byte is
invalid, return an error.
Signed-off-by: NKevin Cernekee <cernekee@gmail.com>
Tested-by: NDave Turvene <dturvene@dahetral.com>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>