usb: gadget: allow working with multiple u_bam_data ports simultaneously

This change introduces a mechanism for distributing port
numbers between the different USB function drivers so
that they can work simultaneously.
Currently, only one instance per USB function is supported.
Working with more than one instance of each USB function
may require additional changes.

Change-Id: I00b4ec068519e14a49a8392dc3b35ba89d1307b8
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>