net: fix vlan validation

VLAN identifiers are 12-bit decimal numbers, not IP addresses.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

include/env_flags.h

@@ -57,8 +57,8 @@ enum env_flags_varaccess {
 	"gatewayip:i," \
 	"netmask:i," \
 	"serverip:i," \
-	"nvlan:i," \
-	"vlan:i," \
+	"nvlan:d," \
+	"vlan:d," \
 	"dnsip:i,"
 #else
 #define ETHADDR_FLAGS