usb: mdm_bridge: Add support for multiple instance of dun and rmnet driver

mdm bridge driver needs to get loaded for two mdm9x15 devices enumeration
for PID 0x9048 and 0x9079. In this case driver probe and disconnect can
happen in parallel on two different cores. Hence identify the available
bridge device instance and mark is as claimed by acquiring a global mutex.
Unclaime the device instance upon device disconnect.

Change-Id: I8f0f8fb7d76e8a4f1a68b3fad288782ff571c653
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>