meth driver renovation

The meth ethernet driver for the SGI IP32 aka O2 is so far still an old
style driver which does not use the device driver model.  This is now
causing issues with some udev based gadgetry in debian-stable.  Fixed by
converting the meth driver to a platform device.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

--
Fixes since previous patch:

  o Fixed typo in meth_exit_module()
Signed-off-by: Jeff Garzik <jeff@garzik.org>