pw_rpc: Use PW_ASSERT in header, not PW_DCHECK
pw_rpc: Use PW_ASSERT in header, not PW_DCHECK

This fixes an issue where RPC headers would include log.h before sources
had set the log module name.

Change-Id: I3b3d9fb937491fdded609aac08403e0708402409
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/93723
Pigweed-Auto-Submit: Scott James Remnant <6b88cbe64f92c5d64ffbf57d7b3d2efc0467fe34@google.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@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