wl18xx: pad only last frame in aggregration buffer for PG2

In PG2 only the last frame in the aggregate buffer should be
aligned to the sdio block size. This frame's header msb should be
set to 0, while in all the previous frames in the aggregation
buffer, this bit should be set to 1.

[Add a HW op for setting the frame ctrl bit only for 18xx. Other minor
cleanups - Arik]

[Make the pre_pkt_send operation optional -- Luca]

Signed-off-by: Ido Reis <idor@ti.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>