usb: gadget: net2280: fix use of GPEP in both directions

USB3380 enhanced mode allows GPEP to be used in both IN and OUT
directions. However, IN and OUT endpoints must use same USB endpoint
address (bEndpointAddress). Fix this by setting the ep_cfg.ep_number
during initialization and keep it in net2280_enable()

Tested-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>