usb: mdm_bridge: Remove platform device before freeing structure

This gives platform drivers a chance to do proper cleanup, especially
if they call ctrl_bridge_close which needs to access several of the
structure's members.

Change-Id: I56f36ff105ec131e560ea8a436a89316f6f88974
Signed-off-by: Jack Pham <jackp@codeaurora.org>