net: dsa: vsc73xx: pass value in phy_write operation

In the 'vsc73xx_phy_write' function, the register value is missing,
and the phy write operation always sends zeros.

This commit passes the value variable into the proper register.

Fixes: 05bd97fc ("net: dsa: Add Vitesse VSC73xx DSA router driver")
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>