samplefmt: add av_get_bytes_per_sample()

Deprecate av_get_bits_per_sample_fmt(), which was a misnamed function.

For the moment we don't have sample formats with a non-integer number
of bytes, in that case we may need to create a new
av_get_bits_per_sample() function. In the meanwhile we prefer to adopt
this variant, since avoids divisions by 8 all over the place.