iwlagn: fix RXON HT

When the HT information is changed due to
BSS changes (like legacy stations joining)
we need to recalculate HT RXON parameters.

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