net: phy: at803x: raname hw_stats functions to qca83xx specific name

The function and the struct related to hw_stats were specific to qca83xx
PHY but were called following the convention in the driver of calling
everything with at803x prefix.

To better organize the code, rename these function a more specific name
to better describe that they are specific to 83xx PHY family.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>