iwlwifi: add debug messages when start aggregation queue

This patch adding few more debug messages if encounter error when driver
try to start tx aggregation queue. Also change from IWL_ERR to
IWL_DEBUG_HT is the HW legacy queue is empty when driver request to move
to aggregation queue.

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