net: sfp: fix sfp_bus_add_upstream() warning

When building with SFP disabled, the stub for sfp_bus_add_upstream()
missed "inline".  Add it.

Fixes: 727b3668 ("net: sfp: rework upstream interface")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>