net: Allow drivers to return -ENOSYS with the write_hwaddr() method

Some drivers may want to implement this method for some of their devices but
not for others. So it is not possible to just leave the operation out of
the table. Drivers could get around this by masquerading as two separate
drivers but that seems unpleasant.

Allow the driver to return an error when it does not want to process the
write_hwaddr() method.

Signed-off-by: Simon Glass <sjg@chromium.org>