cromo: Make sure the modem is registered before connecting.

Also assume StartDataSession succeeds when it returns
kOperationHasNoEffect.

BUG=chromium-os:36171
TEST=cellular_3GModemControl

Change-Id: Ibb7204c4e48b2aca1349890e8a8db558e0ef47cc
Reviewed-on: https://gerrit.chromium.org/gerrit/37786
Reviewed-by: Thieu Le <thieule@chromium.org>
Tested-by: Thieu Le <thieule@chromium.org>
Commit-Ready: Thieu Le <thieule@chromium.org>
3 files changed