net: bridge: consolidate bridge and port linkxstats calls

In the bridge driver we usually have the same function working for both
port and bridge. In order to follow that logic and also avoid code
duplication, consolidate the bridge_ and brport_ linkxstats calls into
one since they share most of their code. As a side effect this allows us
to dump the vlan stats also via the slave call which is in preparation for
the upcoming per-port vlan stats and vlan flag dumping.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>