wireless/p54: Remove duplicated net2280 header

The usb gadget driver net2280 has exported a header file with the
register definition of the net2280 chip.

Remove the custom/duplicated header file in favor of that header file
in include/linux

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>