wl12xx: fix sparse warning about undeclared wl12xx_alloc_dummy_packet

Fix sparse warning:

  CHECK   drivers/net/wireless/wl12xx/main.c
drivers/net/wireless/wl12xx/main.c:1246:17: warning: symbol 'wl12xx_alloc_dummy_packet' was not declared. Should it be static?
Signed-off-by: NLuciano Coelho <coelho@ti.com>

drivers/net/wireless/wl12xx/main.c

@@ -1243,7 +1243,7 @@ int wl1271_tx_dummy_packet(struct wl1271 *wl)
  */
 #define TOTAL_TX_DUMMY_PACKET_SIZE (ALIGN(1400, 512))
 
-struct sk_buff *wl12xx_alloc_dummy_packet(struct wl1271 *wl)
+static struct sk_buff *wl12xx_alloc_dummy_packet(struct wl1271 *wl)
 {
 	struct sk_buff *skb;
 	struct ieee80211_hdr_3addr *hdr;