iwlwifi: mvm: disable dbg data collect when fw isn't alive

If FW isn't alive, trying to collect debug data will
result in errors both in driver and in the collected
data, so just warn and leave the collecting function
in this case.

Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>