The code in port.c and slave.c creating the phylink instance is very
similar - let's consolidate this into a single function.
Signed-off-by: NRussell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: NMarek Behún <kabel@kernel.org>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>