net: phy: adin: add ethtool get_stats support

This change implements retrieving all the error counters from the PHY.

The counters require that the RxErrCnt register (0x0014) be read first,
after which copies of the counters are latched into the registers. This
ensures that all registers read after RxErrCnt are synchronized at the
moment that they are read.

The counter values need to be accumulated by the driver, as each time that
RxErrCnt is read, the values that are latched are the ones that have
incremented from the last read.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>