mac80211: add hw configuration for max ampdu buffer size

Some devices don't support the maximum AMDPU buffer size of 64, so we
need to add an option to configure this in the hardware configuration.
This value will be used in the ADDBA response instead of the value
suggested in the request, if the latter is greater than the max
supported.

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