net: phylink: fix failure to register on x86 systems

The kernel test robot reports a boot failure with qemu in 5.5-rc,
referencing commit 2203cbf2

 ("net: sfp: move fwnode parsing into
sfp-bus layer"). This is caused by phylink_create() being passed a
NULL fwnode, causing fwnode_property_get_reference_args() to return
-EINVAL.

Don't attempt to attach to a SFP bus if we have no fwnode, which
avoids this issue.

Reported-by: kernel test robot <rong.a.chen@intel.com>
Fixes: 2203cbf2

 ("net: sfp: move fwnode parsing into sfp-bus layer")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>