compat: add new way to backport the usage of net_device_ops

compat: add new way to backport the usage of net_device_ops

We tend to #ifdef code that uses the new net_device_ops
on kernels older than 2.6.29. This tends to create a burden
on the upkeeping of the hunks. We can minimize our work by
instead definining a new netdev_attach_ops() which will do
our backport work for the older kernels and simply attach
the ops for newer kernels.

This should simplify backporting of network drivers
considerably.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>