iwlwifi/fw: use struct_size over open coded arithmetic

Replace zero-length array with flexible-array member and make use
of the struct_size() helper in kzalloc().
Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Minghao Chi (CGEL ZTE) <chi.minghao@zte.com.cn>
Link: https://lore.kernel.org/r/20220216030841.1839666-1-chi.minghao@zte.com.cnSigned-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/fw/dbg.c

@@ -2749,7 +2749,7 @@ int iwl_fw_dbg_collect(struct iwl_fw_runtime *fwrt,
 		delay = le32_to_cpu(trigger->stop_delay) * USEC_PER_MSEC;
 	}
 
-	desc = kzalloc(sizeof(*desc) + len, GFP_ATOMIC);
+	desc = kzalloc(struct_size(desc, trig_desc.data, len), GFP_ATOMIC);
 	if (!desc)
 		return -ENOMEM;