usb: gadget: net2280: remove fiforegs as it is unused

Remove fiforegs from struct net2280 and net2280_ep as it is unused.
By the way, ep->fiforegs = &dev->fiforegs[i] assignment is incorrect.
It should be ep->fiforegs = &dev->fiforegs[ne[i]], but it doesn't
matter now.

Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>