Allow the main drivers to specify a custom version of the net_device_ops
structure. This is required by orinoco_usb to supply a separate transmit
function.

Export existing net_device_ops callbacks so that the drivers can reuse
some of the existing code.
Signed-off-by: David Kilroy <kilroyd@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>