staging: vt6656: Use sk_buff buffer for tx header

staging: vt6656: Use sk_buff buffer for tx header

mac80211 can provide space for the driver to put a tx header on
the skb buffer instead coping the entire frame on to a local
buffer with the header.

To use this extra_tx_headroom must be set in mac80211 with the largest
possible header which is struct vnt_tx_buffer.

The driver has 8 possible combinations of tx header size which
are found in vnt_get_hdr_size replacing vnt_mac_hdr_pos.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/7b967bfc-1d4b-4b45-efab-d54f16cca226@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>