net: phylink: allow attaching phy for SFP modules on 802.3z mode

Some SFPs may contain an internal PHY which may in some cases want to
connect with the host interface in 1000base-x/2500base-x mode.
Do not fail if such PHY is being attached in one of these PHY interface
modes.

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