USB: RMNET: smd_ctrl: Fix crash if platform driver register fails

If platform driver register fails on connect, driver still tries
to unregister it on disconnect. This results in crash. Fix this
by not doing pdrv unregister if register had failed.

Change-Id: I999b981941e59285186c6e49cbaafc6adb5a9eb6
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>