net: dcb: add DSCP to comment about priority selector types

Commit ee205981 ("net/dcb: Add dscp to priority selector type")
added a define for the new DSCP selector type created by
IEEE 802.1Qcd, but missed the comment enumerating all selector types.
Update the comment.
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: NPetr Machata <petrm@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/uapi/linux/dcbnl.h

@@ -233,7 +233,8 @@ struct cee_pfc {
  *	2	Well known port number over TCP or SCTP
  *	3	Well known port number over UDP or DCCP
  *	4	Well known port number over TCP, SCTP, UDP, or DCCP
- *	5-7	Reserved
+ *	5	Differentiated Services Code Point (DSCP) value
+ *	6-7	Reserved
  *
  *  Selector field values for CEE
  *	0	Ethertype