net: dsa: warn if phylink_mac_link_state returns error

Issue a warning to the kernel log if phylink_mac_link_state() returns
an error. This should not occur, but let's make it visible.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>