net: sfp: move rtnl lock to cover reading state

As preparation to moving st_mutex inside rtnl_lock, we need to first
move the rtnl lock to cover reading the state.

Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>