sfp: fix module initialisation with netdev already up

It was been observed that with a particular order of initialisation,
the netdev can be up, but the SFP module still has its TX_DISABLE
signal asserted.  This occurs when the network device brought up before
the SFP kernel module has been inserted by userspace.

This occurs because sfp-bus layer does not hear about the change in
network device state, and so assumes that it is still down.  Set
netdev->sfp when the upstream is registered to work around this problem.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>