FwmarkServer - fix a bugprone-branch-clone compiler warning

Fixes:
  system/netd/server/FwmarkServer.cpp:253:9: warning: switch has 3 consecutive identical branches [bugprone-branch-clone]

Test: builds
Bug: 153035880
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I78aa7f7ebe2541095df1bba3354fbb9b8365ae1c