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>