iwlwifi: remove redundant argument from iwl_dump_nic_event_log

We can check buf against NULL instead of having additional bool
variable.

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