usb: gadget: Support multiple HSIC transport instances for dun and rmnet

Instead of using integer-based IDs to couple bridge instances to the
corresponding gadget ports, assign transport name strings (passed by
userspace) describing the port instance, and match strings to identify
the correct instance when bridge_open() called. This allows multiple
dun and rmnet ports to be assigned without having dependecy of
one-to-one mapping between gadget port instance and bridge driver
instance.

Change-Id: Ia87bd6e9c72d5e98f47dbc2a1cf0b71b48daa1a8
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>