linkmode: introduce linkmode_intersects()

Add a new helper to find intersections between Ethtool link modes,
linkmode_intersects(), similar to the other linkmode helpers.

Signed-off-by: Alexander Lobakin <alobakin@marvell.com>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>