mac80211: allow drivers to set default uAPSD parameters

mac80211 currently sets uAPSD parameters to have VO AC trigger-
and delivery-enabled, with maximum service period length.

Allow drivers to change these default settings since different
uAPSD client implementations may handle errors differently and
be able to recover from some errors.

Note: some APs may not function correctly if one or all ACs are
trigger- and delivery-enabled, see
http://thread.gmane.org/gmane.linux.kernel.wireless.general/93577

.
We retested with this AP and later firmware doesn't have this
bug any more.

Signed-off-by: Alexander Bondar <alexander.bondar@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>