iwlwifi: don't WARN on trying to dump dead firmware

There's no point in warning here, the user will just get an
error back to the debugfs file write, and warning just makes
it seem like there's an internal consistency problem when in
reality the user just happened to hit this at a bad time.
Remove the warning.

Fixes: f45f979d ("iwlwifi: mvm: disable dbg data collect when fw isn't alive")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>