wl1251: use skb_trim to make skb shorter

the current code is directly setting skb->len, which is not correct and
brings problems with HAVE_EFFICIENT_UNALIGNED_ACCESS enabled in config
Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov.75@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ti/wl1251/rx.c

@@ -180,7 +180,7 @@ static void wl1251_rx_body(struct wl1251 *wl,
 	wl1251_mem_read(wl, rx_packet_ring_addr, rx_buffer, length);
 
 	/* The actual length doesn't include the target's alignment */
-	skb->len = desc->length  - PLCP_HEADER_LENGTH;
+	skb_trim(skb, desc->length - PLCP_HEADER_LENGTH);
 
 	fc = (u16 *)skb->data;