wlcore: fix size of two memset's in wl1271_cmd_build_arp_rsp()

We currently do this:

int wl1271_cmd_build_arp_rsp(struct wl1271 *wl, struct wl12xx_vif *wlvif)
...
      struct wl12xx_arp_rsp_template *tmpl;
      struct ieee80211_hdr_3addr *hdr;
...
      tmpl = (struct wl12xx_arp_rsp_template *)skb_put(skb, sizeof(*tmpl));
      memset(tmpl, 0, sizeof(tmpl));
...
      hdr = (struct ieee80211_hdr_3addr *)skb_push(skb, sizeof(*hdr));
      memset(hdr, 0, sizeof(hdr));
...

I believe we want to set the entire structures to 0 with those
memset() calls, not just zero the initial part of them (size of the
pointer bytes).

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Luciano Coelho <coelho@ti.com>