mxfdec: fix Avid AirSpeed files being misinterpreted as OP1a

The "ECs != 1 -> OP1a" assumption was wrong. Luckily, the file
that triggered that behavior had two ECs, not zero. Hence
distinguishing between them is simple in this case.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>