net: wwan: iosm: remove pointless null check

net: wwan: iosm: remove pointless null check

GCC 12 warns:

drivers/net/wwan/iosm/iosm_ipc_protocol_ops.c: In function ‘ipc_protocol_dl_td_process’:
drivers/net/wwan/iosm/iosm_ipc_protocol_ops.c:406:13: warning: the comparison will always evaluate as ‘true’ for the address of ‘cb’ will never be NULL [-Waddress]
  406 |         if (!IPC_CB(skb)) {
      |             ^

Indeed the check seems entirely pointless. Hopefully the other
validation checks will catch if the cb is bad, but it can't be
NULL.

Reviewed-by: M Chetan Kumar <m.chetan.kumar@intel.com>
Link: https://lore.kernel.org/r/20220519004342.2109832-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>