net: sfp: use linkmode_*() rather than open coding

Use the linkmode_*() helpers rather than open coding the calls to the
bitmap operators.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>