net: dsa: vsc73xx: use defined values in phy operations

This commit changes magic numbers in phy operations.
Some shifted registers was replaced with bitfield macros.

No functional changes done.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>