usb: gadget: mbim: Add support for MS feature descriptors

Windows hosts support dynamically selecting a configuration that
contains the MBIM function, a feature Microsoft calls "Identity
Morphing". This is supported by responding to OS-specific feature
descriptor requests, in which the subCompatibleID field is populated
with the configuration number that contains the MBIM function.

Change-Id: I8a0132342cc7601ba7ad81070ae3808552957bc4
Signed-off-by: Jack Pham <jackp@codeaurora.org>