net: dsa: mv88e6xxx: Be explicit about DT or pdata

Make it explicit that either device tree is used or platform data.  If
neither is available, abort the probe.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 877b7cb0 ("net: dsa: mv88e6xxx: Add minimal platform_data support")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>