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>