staging: vt6656: use NULLFUCTION stack on mac80211

It appears that the drivers does not go into power save correctly the
NULL data packets are not being transmitted because it not enabled
in mac80211.

The driver needs to capture ieee80211_is_nullfunc headers and
copy the duration_id to it's own duration data header.

Cc: stable <stable@vger.kernel.org>
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/610971ae-555b-a6c3-61b3-444a0c1e35b4@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>