net: dsa: loop: Implement ethtool statistics

When a DSA driver implements ethtool statistics, we also override the
master network device's ethtool statistics with the CPU port's
statistics and this has proven to be a possible source of bugs in the
past. Enhance the dsa_loop.c driver to provide statistics under the
forme of ok/error reads and writes from the per-port PHY read/writes.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>