net: phylink: add phylink_set_10g_modes() helper

Add a helper for setting 10Gigabit modes, so we have one central
place that sets all appropriate 10G modes for a driver.

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