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>
diff --git a/drivers/net/wireless/ti/wl1251/rx.c b/drivers/net/wireless/ti/wl1251/rx.c
index 123c4bb..cde0eaf 100644
--- a/drivers/net/wireless/ti/wl1251/rx.c
+++ b/drivers/net/wireless/ti/wl1251/rx.c
@@ -180,7 +180,7 @@
 	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;