iwlwifi: don't send REPLY_REMOVE_ALL_STA upon exit

This patch avoids sending REPLY_REMOVE_ALL_STA in down flow, this avoids a
meaningless warning from being printed
On the way this patch also renames the the function to
iwl_clear_stations_table

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>