iwlwifi: Fix TKIP update key and get_free_ucode_key

This patch fixes a bug in update_tkip_key: only one key needs to be
allocated in uCode, every time it is updated, the old one will be
overwritten

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