dpif-netdev: Correct type of struct dp_netdev_flow's 'tcp_flags' member.

dpif-netdev: Correct type of struct dp_netdev_flow's 'tcp_flags' member.

TCP flags fit in 8 bits so this type seems more appropriate.

Fixes the following "sparse" warnings introduced by commit 734ec5ec1349
(packet: Add additional TCP flags extraction on IPv6.):

dpif-netdev.c:630: warning: incorrect type in assignment (different base types)
dpif-netdev.c:630:    expected unsigned char [unsigned] [usertype] tcp_flags
dpif-netdev.c:630:    got restricted __be16 [usertype] tcp_flags
dpif-netdev.c:979: warning: invalid assignment: |=
dpif-netdev.c:979:    left side has type restricted __be16
dpif-netdev.c:979:    right side has type unsigned char

CC: Jesse Gross <jesse@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>