iwlwifi: remove sync_cmd_mutex

We now use priv->mutex to serialize sync command, remove old
priv->sync_cmd_mutex and add assertion that priv->mutex must be locked.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>