net: wwan: iosm: fix kernel test robot reported error

sparse warnings - iosm_ipc_mux_codec.c:1474 using plain
integer as NULL pointer.

Use skb_trim() to reset skb tail & len.

Fixes: 9413491e ("net: iosm: encode or decode datagram")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: M Chetan Kumar <m.chetan.kumar@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wwan/iosm/iosm_ipc_mux_codec.c

@@ -1471,8 +1471,7 @@ void ipc_mux_ul_encoded_process(struct iosm_mux *ipc_mux, struct sk_buff *skb)
 			ipc_mux->ul_data_pend_bytes);
 
 	/* Reset the skb settings. */
-	skb->tail = 0;
-	skb->len = 0;
+	skb_trim(skb, 0);
 
 	/* Add the consumed ADB to the free list. */
 	skb_queue_tail((&ipc_mux->ul_adb.free_list), skb);