USB: mdm_bridge: Fix probe error path

The platform device is not unregistered in probe error path.  The
platform device is added again during next probe and it fails.  But
the return value is not checked.  Kernel panic happens when platform
device is removed during disconnect.

CRs-Fixed: 465508
Change-Id: I4db613e72d8b405f10cde0d1335d231cf5079e64
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>