codec_opus: Fix warning when Opus negotiated but codec_opus not loaded.

When Opus is negotiated but not loaded, the log is spammed with messages
because the system does not know how to calculate the number of samples in
a frame.

* Suppress the warning by supplying a function that assumes 20ms of
samples in the frame.  For pass through support it doesn't really seem to
matter what number of samples is returned anyway.

ASTERISK-26605 #close

Change-Id: Icf2273692f040dc2c45b01e72a790d11092f9e0f