media: mceusb: sanity check for prescaler value

prescaler larger than 8 would mean the carrier is at most 152Hz,
which does not make sense for IR carriers.

Reported-by:  <syzbot+6d31bf169a8265204b8d@syzkaller.appspotmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>