libopencore-amr, libvo-amrwbenc: Only check the bitrate when changed

Also rename the incorrectly named enc_bitrate to enc_mode, use the
enc_bitrate variable for storing the last chosen bitrate.

This avoids continuous warning log messages if not using an
exactly matching bitrate, while still allowing changing bitrate
at any point.

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