iwlwifi: lib: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, change the following form:

sizeof(*pattern_cmd) +
               wowlan->n_patterns * sizeof(struct iwlagn_wowlan_pattern)

 to :

struct_size(pattern_cmd, patterns, wowlan->n_patterns)

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/dvm/lib.c

@@ -1009,8 +1009,7 @@ int iwlagn_send_patterns(struct iwl_priv *priv,
 	if (!wowlan->n_patterns)
 		return 0;
 
-	cmd.len[0] = sizeof(*pattern_cmd) +
-		wowlan->n_patterns * sizeof(struct iwlagn_wowlan_pattern);
+	cmd.len[0] = struct_size(pattern_cmd, patterns, wowlan->n_patterns);
 
 	pattern_cmd = kmalloc(cmd.len[0], GFP_KERNEL);
 	if (!pattern_cmd)