net: phylink: add phylink_link_is_up() helper

Add a helper to determine whether the link is up or down. Currently
this is only used in one location, but becomes necessary to test
when reconfiguring EEE.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/E1tYADl-0014Ph-EV@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>