mwifiex: use struct_size() in kzalloc()

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

struct foo {
    int stuff;
    struct boo entry[];
};

size = sizeof(struct foo) + count * sizeof(struct boo);
instance = kzalloc(size, GFP_KERNEL)

Instead of leaving these open-coded and prone to type mistakes, we can
now use the new struct_size() helper:

instance = kzalloc(struct_size(instance, entry, count), GFP_KERNEL)

Notice that, in this case, variable regd_size is not necessary,
hence it is removed.

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/marvell/mwifiex/sta_cmdresp.c

@@ -1025,17 +1025,14 @@ mwifiex_create_custom_regdomain(struct mwifiex_private *priv,
 	struct ieee80211_regdomain *regd;
 	struct ieee80211_reg_rule *rule;
 	bool new_rule;
-	int regd_size, idx, freq, prev_freq = 0;
+	int idx, freq, prev_freq = 0;
 	u32 bw, prev_bw = 0;
 	u8 chflags, prev_chflags = 0, valid_rules = 0;
 
 	if (WARN_ON_ONCE(num_chan > NL80211_MAX_SUPP_REG_RULES))
 		return ERR_PTR(-EINVAL);
 
-	regd_size = sizeof(struct ieee80211_regdomain) +
-		    num_chan * sizeof(struct ieee80211_reg_rule);
-
-	regd = kzalloc(regd_size, GFP_KERNEL);
+	regd = kzalloc(struct_size(regd, reg_rules, num_chan), GFP_KERNEL);
 	if (!regd)
 		return ERR_PTR(-ENOMEM);