Add missing return statement to Run() in GobiQDLService/Main.cpp

BUG=chromium:690629
TEST=Run unit tests and network3g tests with Gobi3k modem.

Change-Id: I6bc58aed47364022b323a558dd028d67052cf0cf
Reviewed-on: https://chromium-review.googlesource.com/440370
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>