iwlwifi: dvm: don't send HCMD in restart flow

There is a race between the restart flow and the workers.
The workers are cancelled after the fw is already killed
and might send HCMD when there is fw to handle them.
Simply check that there is a fw to which the HCMD can be
sent before actually sending it.

Cc: stable@vger.kernel.org
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>