iwlwifi: don't update TFD free counter for invalid station

This patch makes sure that station table is not accessed
with invalid station id in 4965 TX response path

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>