usb: gadget: Add support for multiple RmNET

Extend number of supported rmnet function instances from 1 to 3.
This is done in order to fulfill the requirement for simultaneous
connection and work of several rmnet interfaces on 9615.
Other targets are not affected.

Change-Id: Ib830da2d6b21e57b107de4c90e3018e8d45be160
Signed-off-by: Anna Perel <aperel@codeaurora.org>