wifi: mwifiex: Fix the size of a memory allocation in mwifiex_ret_802_11_scan()

The type of "mwifiex_adapter->nd_info" is "struct cfg80211_wowlan_nd_info",
not "struct cfg80211_wowlan_nd_match".

Use struct_size() to ease the computation of the needed size.

The current code over-allocates some memory, so is safe.
But it wastes 32 bytes.

Fixes: 7d7f07d8 ("mwifiex: add wowlan net-detect support")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/7a6074fb056d2181e058a3cc6048d8155c20aec7.1683371982.git.christophe.jaillet@wanadoo.fr