iwlwifi: lib: 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(*pattern_cmd) +
               wowlan->n_patterns * sizeof(struct iwlagn_wowlan_pattern)

 to :

struct_size(pattern_cmd, patterns, wowlan->n_patterns)

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>