net: sfp: fix hwmon

The referenced commit below allowed more than one hwmon device to be
created per SFP, which is definitely not what we want. Avoid this by
only creating the hwmon device just as we transition to WAITDEV state.

Fixes: 139d3a21 ("net: sfp: allow modules with slow diagnostics to probe")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>