Correctly implement the OpenFlow 1.2+ OXM_OF_IP_DSCP field.

Correctly implement the OpenFlow 1.2+ OXM_OF_IP_DSCP field.

NXM puts the DSCP value in bits 2-7 of NXM_OF_IP_TOS.
OXM puts the DSCP value in bits 0-6 of OXM_OF_IP_DSCP.

Before this commit, Open vSwitch incorrectly implemented OXM_OF_IP_DSCP
with the same format as NXM_OF_IP_TOS.  This commit fixes the problem and
adds a test (previously missing but I don't know why).

Reported-by: Hiroshi Miyata <miyahiro.dazu@gmail.com>
Tested-by: Hiroshi Miyata <miyahiro.dazu@gmail.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>