mac80211: enable starting BA session with custom timeout

mac80211: enable starting BA session with custom timeout

Currently the debugfs entry for starting aggregation session
starts it with timeout of 5 seconds. Allow opening a session
with a custom timeout (according to spec 0 is no timeout).
while at it, refactor the function and remove the magic numbers.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>