net: phy: bcm7xxx: Plug in support for reading PHY error counters

Broadcom BCM7xxx internal PHYs can leverage the Broadcom PHY library
module PHY error counters helper functions, just implement the
appropriate PHY driver function calls to do so. We need to allocate some
storage space for our PHY statistics, and provide it to the Broadcom PHY
library, so do this in a specific probe function, and slightly wrap the
get_stats function call.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>