[PATCH V2] wcn36xx: use struct_size over open coded arithmetic

[cgel.zte at gmail.com]

change log:

v1: msg_ind = kmalloc(struct_size(*msg_ind, msg, len), GFP_ATOMIC);
v2: msg_ind = kmalloc(struct_size(msg_ind, msg, len), GFP_ATOMIC);

thanks
Minghao