net: phy: aquantia: add SGMII statistics

The AQR107 family has SGMII statistics counters. Let's expose them to
ethtool. To interpret the counters correctly one has to be aware that
rx on SGMII side is tx on ethernet side. The counters are populated
by the chip in 100Mbps/1Gbps mode only.

v2:
- add constant AQR107_SGMII_STAT_SZ
- add struct aqr107_priv to be prepared for more private data fields
- let aqr107_get_stat() return U64_MAX in case of an error

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>