net: add linkmode helper for setting flow control advertisement

Add a linkmode helper to set the flow control advertisement in an
ethtool linkmode mask according to the tx/rx capabilities. This
implementation is moved from phylib, and documented with an
analysis of its shortcomings.

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>