usb: gadget: udc: net2280: Remove function ep_stall

irqs_superspeed calls ep_stall instead of set/clear_halt, due to a
workaround for SS seqnum. Create a function with the workaround and
call set/clear_halt instead.

This way we can compare the code of super/normal speed and it is easier
to follow the code.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>