iwlwifi: tx/rx queue pointer information

Adding debugfs function to show current TxFifo/RxFifo read/write
pointer, plus the current tx queue status (wake/stop) for both real and
virtual queue.
This is part of debug feature set to help debugging driver/uCode.

use tx_queue and rx_queue in
/sys/kernel/debug/ieee80211/phy0/iwlagn/debug directory to show the
current read/write pointer for both TxFifo and RxFifo queue

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>