rtpbitrateadapter: should make no adaption by default

Description of "bitrate" property says 0 (the default value) means
the element performs no adaption, and so one would assume it would
remain passive until "bitrate" is set to some nonzero value. However,
when "bitrate" is left unset, the adapter instead requests video in tiny
128x96 resolution on its sink pad.

In order for fs_rtp_bitrate_adapter_getcaps() to return peer_caps by
default, the value of FsRtpBitrateAdapter::bitrate has to be initialized
to G_MAXUINT.

Also fix the comments to say that MAXUINT is no adaptation.

https://bugs.freedesktop.org/show_bug.cgi?id=99183