net: phy: aquantia: replace magic numbers with constants

Replace magic numbers with proper constants. The original patch is
from Andrew, I extended / adjusted certain parts:
- Use decimal bit numbers. The datasheet uses hex bit numbers 0 .. F.
- Order defines from highest to lowest bit numbers
- correct some typos
- add constant MDIO_AN_TX_VEND_INT_MASK2_LINK
- Remove few functional improvements from the patch, they will come as
  a separate patch.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>