staging: vt6656: Use mac80211 duration for tx headers

mac80211 already provides the correct duration simply
copy it to the tx headers removing the need for driver
to find it with vnt_get_duration_le.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/0cdde611-119b-b223-e8c8-b59fb497a7b9@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>