rtpdec: Interpret the different G726 names as bits_per_coded_sample

rtpdec: Interpret the different G726 names as bits_per_coded_sample

For the standardized 8 kHz sample rate, this works exactly the same.
For nonstandard sample rates, the different predefined G726
names (G726-16, G726-24, G726-32, G726-40) are interpreted as an
indication of the bits per coded sample, even though their
actual bitrates aren't what the name specifies.

This feels more sane than using free-form names for nonstandard
sample rate/bitrate combinations, e.g like G726-22, G726-33
for 11025 Hz.

Signed-off-by: Martin Storsjö <martin@martin.st>