net: dsa: vsc73xx: fix port MAC configuration in full duplex mode

According to the datasheet description ("Port Mode Procedure" in 5.6.2),
the VSC73XX_MAC_CFG_WEXC_DIS bit is configured only for half duplex mode.

The WEXC_DIS bit is responsible for MAC behavior after an excessive
collision. Let's set it as described in the datasheet.

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>