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.
Signed-off-by: Pawel Dembicki <paweldembicki@xxxxxxxxx>