libavformat/mtv: make clear we assume bpp is always 16

All samples in the wild are RGB565/555 and we are already
acting on this assumption when pushing out the video frames,
so if we get anything != than 16 for bpp we just override
this value for doing any calculations making our approach
consistent.

Also avoid repeatedly shifting bpp.

Signed-off-by: Reynaldo H. Verdejo Pinochet <r.verdejo@sisa.samsung.com>
