During system boot, it shows a warning message:
"dl_attach: DL_ERROR_ACK bad PPA". But system works fine after that.
The problem is that softmac_mac_register() calls the dl_attach() function and
tries to determine whether the underlying driver support VLAN ppa hack. It is
perfect fine that if the driver cannot, but it is annoying to see the warning
message "dl_attach: DL_ERROR_ACK bad PPA"