ath6kl: debug: 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(*tbl) + num_entries * sizeof(struct wmi_bss_roam_info)

 to :

struct_size(tbl, info, num_entries)

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>