wl12xx: add hw configuration for max supported AMDPU size

The wl12xx chips do the AMDPU aggregation work in the firmware, but it
supports a maximum of 8 frames per block.  Configure the mac80211 hw
structure accordingly.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Tested-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>