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

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

[ Upstream commit 985a02e75881b73a43c9433a718b49d272a9dd6b ]

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

Use skb_trim() to reset skb tail & len.

Fixes: 9413491e20e1 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>