net: phy: at803x: make read_status more generic

Make read_status more generic in preparation on moving it to shared
library as other PHY Family Driver will have the exact same
implementation.

The only specific part was a check for AR8031/33 if 1000basex was used.
The check is moved to a dedicated function specific for those PHYs.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>