blackfin: Initialize the EMAC VLAN with proper default value

EMAC_VLANx regs is not properly initiallized in u-boot, once it's overwrite in the
kernel when DSA enabled, hot reset will lead to bringing up EMAC fail in u-boot.

Signed-off-by: Aaron Wu <Aaron.Wu@analog.com>
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>