usb: gadget: ether: allocate and init otg descriptor by otg capabilities

Allocate and initialize usb otg descriptor according to gadget otg
capabilities, add it for each usb configurations, free it while
ether unbind. If otg capability is not defined, keep its otg
descriptor unchanged.

Signed-off-by: Li Jun <jun.li@freescale.com>
Reviewed-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>