iwlagn: enable 11n aggregation without checking traffic load

Enable HT aggregation when it reach reasonable traffic without
checking traffic load which delay enabling the aggregation and lower
the throughput

but this behavior can be overwrite by module parameter

this address
https://bugzilla.kernel.org/show_bug.cgi?id=40042



Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>