net: iosm: remove an unnecessary NULL check

The address of &ipc_mux->ul_adb can't be NULL because it points to the
middle of a non-NULL struct.

Fixes: 9413491e

 ("net: iosm: encode or decode datagram")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: M Chetan Kumar <m.chetan.kumar@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>