net: phylink: Add phylink_mac_link_{up, down} wrapper functions

This is a cosmetic patch that reduces the clutter in phylink_resolve
around calling the .mac_link_up/.mac_link_down driver callbacks.  In a
further patch this logic will be extended to emit notifications in case
a net device does not exist.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>