usb: gadget: net2280: fix endpoint max packet for super speed connections

This patch fixes the register offset used for super-speed connection's
max packet size. Without it using the 338x series of devices in enhanced
mode will only allow full or high speed operation to function correctly.

Signed-off-by: Simon Appleby <simon.appleby@pickeringtest.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>