res_format_attr_opus: Update to latest RFC 7587.

Beside that, the format-attribute module sends only non-default values in the
line fmtp, now. This avoids unnecessary overhead in SDP messages. Furthermore,
previously the parameter stereo was not parsed when being the first parameter.

ASTERISK-25583 #close

Change-Id: Iae85ba3e5960bfd5d51cf65bcffad00dd4875a73