iwlwifi: add mac80211 flush callback support

Adding flush callback support in the driver. Two type of flush can be
issued by mac80211:
1. drop = true: frame drop is ok, issue REPLY_TXFIFO_FLUSH host command
to uCode to drop all the frames in tx fifo queues; then return the
control back to mac80211
2. drop = false: wait for either all the frames in tx fifo queues been
transmitted, or timeout; then return the control back to mac80211

If the flush request coming from mac80211, mac80211 will make sure there
are no additional frames push down to driver before flush operation is
completed.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>