iwlagn: finer-grained HT disable

At least while debugging, a lot of people use
11n_disable=1 to disable HT completely. To be
able to figure out what parts of HT cause the
problems we see, make the parameter a bitmap,
allowing to disable all of HT and aggregation
(TX/RX) separately.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>