staging: vt6656: vnt_get_rtscts_duration_le use ieee80211_rts_duration

use the mac80211 ieee80211_rts_duration for RTS frames.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/377a4cc3-cfe3-91aa-cf71-1063f311426a@gmail.com


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