iwlwifi: don't WARN on bad firmware state

When we restart firmware and it is marked as not alive, we can still get
calls from mac80211. Don't WARN on in this situation as this triggers
automatic bug reports with no valuable information.

This continuation of:

commit 8ca95995
Author: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Date:   Sun Sep 15 11:37:17 2013 +0300

    iwlwifi: don't WARN on host commands sent when firmware is dead

which remove WARN_ONCE from one place, but those warnings are also
triggered from other functions.

Patch also adds unlikely() statement.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>