mac80211: stop only the queues assigned to the vif during channel switch

Instead of stopping all the hardware queues during channel switch,
which is especially bad when we have large CSA counts, stop only the
queues that are assigned to the vif that is performing the channel
switch.

Additionally, check for (sdata->csa_block_tx) instead of calling
ieee80211_csa_needs_block_tx(), which can now be removed.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>