net: dsa: Use mnemonics rather than register numbers

Rather than refer to registers by number, define mnemonics. Also
define mnemonics for the commonly used bits within the registers.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>