[media] mceusb: move check earlier to make smatch happy

Smatch complains that "cmdbuf[cmdcount - length]" might go past the end
of the array.  It's an easy warning to silence by moving the limit
check earlier.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>