netdev-linux: favor netlink stats for physical ports

Currently physical ports stats are collected from kernel datapath.
However, those counter do not reflect actual wire packet counters
when GSO, TSO or GRO are enabled by the NIC. In the meantime, the
stats collected form routing stack does. While both stats are valid,
Reporting kernel netdev stats for packet counts and byte counts make
it easier to correlate those numbers with external measurements.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>