usb: gadget: add MBIM support for super-speed function suspend

The USB 3.0 specification defines a new 'Function Suspend' feature.
This feature enables the USB host to put inactive composite device
functions in a suspended state even if the device itself is not
suspended.
This patch adds support for this feature in the MBIM function and
the transport layer.

Change-Id: I0ff9a2af5ddc94679087f5a21f7740adfb1ce708
Signed-off-by: Danny Segal <dsegal@codeaurora.org>