pw_rpc: Use span copy constructor instead of iterator-based one
pw_rpc: Use span copy constructor instead of iterator-based one

Change-Id: I09083a3d4f47aa7a7c6781710f26bb3d4a38e748
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/99287
Reviewed-by: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Pigweed-Auto-Submit: Dennis Kormalev <6222e5e705620133a24041cba003e993480020a9@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed