We can indicate IEEE80211_HW_HAS_RATE_CONTROL to mac80211 because
the hardware has its own rate control algorithm. And what driver needs
to do is to choose an RA mask according the peer's capabilities.

But the hardware is not able to setup BA session by itself. So driver
requires to initiate tx BA session for hardware, and tells it if it is
possible to transmit AMPDU. The hardware can then aggregate MPDUs.

And the size of AMPDU is controlled by the TX descriptor and the
register value. Since the TX descriptor will reference the max AMPDU
size from ieee80211_sta::ht_cap::ampdu_factor, just set the register
value to 0x3f, and let it be controlled by TX descriptor.
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>