net: linkmode: add linkmode_fill() helper

Add a linkmode_fill() helper, which will allow us to convert phylink's
open coded bitmap_fill() operations.

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>