shill: don't auto-connect to a disconnected service

This addresses part of the problem with the 003AsciiKeyWEP40
failure. That test establishes a connection, disconnects,
reconfigures the (WEP) password on an AP, and then tries
to reconnect.

Before this patch, the test failed as follows:
1. shill immediately tries to reconnect after the disconnect.
   This reconnect uses the old password (because the test
   hasn't had a chance to supply a new password).
2. The test code provides the correct password, and then
   initiates an explicit connect. However, the explicit
   connect is ignored, because a connection is already in
   progress (to the same service).
3. The automatic reconnect fails, because it is using the
   wrong password.

BUG=chromium-os:25153,chromium-os:25157
TEST=unit tests, WiFiRoaming, WiFiSecMat.*WEP*, WiFiManager

Testing notes:
- WiFiRoaming had some failures, but no regressions
- WiFiSecMat.*WEP* passed except 003CheckWEP_8021x
  (previously, that failed in addition to 000StaticKeyWEP40,
  001StaticKeyWEP104, 005SharedKeyWEP40, and 006SharedKeyWEP104.)
- WiFiManager passed
  (previously, 003AsciiKeyWEP40 and 004AsciiKeyWEP104 failed.)

Change-Id: I2dd6312006865c914d4193ce3f7d7c5443b84deb
Reviewed-on: https://gerrit.chromium.org/gerrit/14408
Commit-Ready: mukesh agrawal <quiche@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: mukesh agrawal <quiche@chromium.org>
diff --git a/cellular_service.cc b/cellular_service.cc
index c79329a..43aa917 100644
--- a/cellular_service.cc
+++ b/cellular_service.cc
@@ -48,11 +48,10 @@
 CellularService::~CellularService() { }
 
 void CellularService::Connect(Error *error) {
+  Service::Connect(error);
   cellular_->Connect(error);
 }
 
-void CellularService::Disconnect(Error */*error*/) { }
-
 void CellularService::ActivateCellularModem(const string &carrier,
                                             Error *error) {
   cellular_->Activate(carrier, error);