net: phy: icplus: add PHY counter for IP101G

The IP101G provides three counters: RX packets, CRC errors and symbol
errors. The error counters can be configured to clear automatically on
read. Unfortunately, this isn't true for the RX packet counter. Because
of this and because the RX packet counter is more likely to overflow,
than the error counters implement only support for the error counters.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>