usb: gadget: composite: Add function to get descriptors

There are a couple places in the code that get the function descriptors
based on the speed. Move this lookup into a function call and add
support to handle the SuperSpeedPlus descriptors as well.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>