Use first_ssrc() instead of ssrcs[0] in channel.cc

This is just a btw change that didn't fit with some other CLs, but a
thing will occasionally distract while not exactly fitting in with other
changes. :)

Bug: none
Change-Id: Ib15aee691478da8d4b7669009fce963165b7925e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/460580
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Auto-Submit: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47273}