wimax/i2400m: 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, replace code of the following form:

sizeof(*tx_msg) + le16_to_cpu(tx_msg->num_pls) * sizeof(tx_msg->pld[0]);

with:

struct_size(tx_msg, pld, le16_to_cpu(tx_msg->num_pls));

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wimax/i2400m/tx.c

@@ -640,8 +640,7 @@ void i2400m_tx_close(struct i2400m *i2400m)
 	 * figure out where the next TX message starts (and where the
 	 * offset to the moved header is).
 	 */
-	hdr_size = sizeof(*tx_msg)
-		+ le16_to_cpu(tx_msg->num_pls) * sizeof(tx_msg->pld[0]);
+	hdr_size = struct_size(tx_msg, pld, le16_to_cpu(tx_msg->num_pls));
 	hdr_size = ALIGN(hdr_size, I2400M_PL_ALIGN);
 	tx_msg->offset = I2400M_TX_PLD_SIZE - hdr_size;
 	tx_msg_moved = (void *) tx_msg + tx_msg->offset;