wl1271: Support firmware TX packet aggregation

Instead of sending one packet at a time to the firmware, try to
send all available packets at once.
This optimization decreases the number of transactions, which saves
CPU cycles and increases network throughput.

Signed-off-by: Ido Yariv <ido@wizery.com>
Tested-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>