dp-packet: Reset DPDK hwol flags on init.

dp-packet: Reset DPDK hwol flags on init.

Reset the DPDK hwol flags in dp_packet_init_.  The new hwol bad checksum
flag is uninitialized for non-dpdk ports and this is noticed as test
failures using netdev-dummy ports, when built with the --with-dpdk
flag set. Hence, in this case, packets may be falsely marked as having a
bad checksum. The existing APIs are simplified at the same time by
making them specific to either DPDK or otherwise; they also now
manage a single field.

Reported-at: https://mail.openvswitch.org/pipermail/ovs-discuss/2017-August/045081.html
Fixes: 7451af618e0d ("dp-packet : Update DPDK rx checksum validation functions.")
CC: Sugesh Chandran <sugesh.chandran@intel.com>
Signed-off-by: Darrell Ball <dlu998@gmail.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>