mac80211: use maximum number of AMPDU frames as default in BA RX

When the buffer size is set to zero in the block ack parameter set
field, we should use the maximum supported number of subframes.  The
existing code was bogus and was doing some unnecessary calculations
that lead to wrong values.

Thanks Johannes for helping me figure this one out.

Cc: stable@kernel.org
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Luciano Coelho <coelho@ti.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>