aacsbr: reduce element type mismatch warning severity

All HE-AAC samples with an LFE channel make this warning get spammed on
every frame. Turning off SBR for LFE channels makes sense (since it has
much less coefficients than normal channels do), so this error print is
of no value in this case.
It makes sense to keep the error in other cases, hence why it's still
around, degraded to warning severity since the decoder will still
attempt to decode without SBR.

Signed-off-by: Rostislav Pehlivanov <atomnuker@gmail.com>