[vlan] Treat VLAN 0 as valid

VLAN headers are allowed to contain a VLAN tag of zero, indicating
that the header specifies only a priority and that the packet does not
belong to any VLAN.  The easiest way to handle this is to treat VLAN 0
as being a normal VLAN.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/vlan.h

  * @v tag		VLAN tag
  * @ret is_valid	VLAN tag is valid
  */
-#define VLAN_TAG_IS_VALID( tag ) ( ( (tag) >= 1 ) && ( (tag) < 0xfff ) )
+#define VLAN_TAG_IS_VALID( tag ) ( (tag) < 0xfff )
 
 /**
  * Check VLAN priority is valid