sfc: Fix synchronisation of efx_mtd_{probe,rename,remove}

Currently efx_mtd_rename() can race with the probe() and remove()
functions.

Move probe() before device registration and remove() after
unregistration.  Move initialisation/update of all names based on the
netdev name into a new function and call it under the RTNL immediately
after registration.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>