mv88e6xxx: Fix uninitialized variable warning.

In mv88e6xxx_probe(), ("np" or "pdata") might be an invariant
but GCC can't see that, therefore:

drivers/net/dsa/mv88e6xxx/chip.c: In function ‘mv88e6xxx_probe’:
drivers/net/dsa/mv88e6xxx/chip.c:4420:13: warning: ‘compat_info’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  chip->info = compat_info;

Actually, it should have warned on the "if (!compat_info)" test, but
whatever.

Explicitly initialize to NULL in the variable declaration to
deal with this.

Fixes: 877b7cb0 ("net: dsa: mv88e6xxx: Add minimal platform_data support")
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/mv88e6xxx/chip.c

@@ -4382,9 +4382,9 @@ static const void *pdata_device_get_match_data(struct device *dev)
 static int mv88e6xxx_probe(struct mdio_device *mdiodev)
 {
 	struct dsa_mv88e6xxx_pdata *pdata = mdiodev->dev.platform_data;
+	const struct mv88e6xxx_info *compat_info = NULL;
 	struct device *dev = &mdiodev->dev;
 	struct device_node *np = dev->of_node;
-	const struct mv88e6xxx_info *compat_info;
 	struct mv88e6xxx_chip *chip;
 	int port;
 	int err;