sfc: Move MTD probe after netdev registration and name allocation

The MTD partition is named based on the netdev name, which is set to
'eth%d' before registration.  Also, the MTD partition will currently
be left registered if netdev registration fails.

Fix both these problems by moving the MTD probe after netdev
registration.  Hold the RTNL to serialise this with the netdev
notifier that calls efx_mtd_rename().

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