The imx_scu_call_rpc function returns the result inside the
same "msg" struct containing the transmitted message. This is
implemented by holding a pointer to msg (which is usually on the stack)
in sc_imx_rpc and writing to it from imx_scu_rx_callback.
This means that if the have_resp parameter is incorrect or SCU sends an
unexpected response for any reason the most likely result is kernel stack
corruption.
Fix this by only setting sc_imx_rpc.msg for the duration of the
imx_scu_call_rpc call and warning in imx_scu_rx_callback if unset.
Print the unexpected response data to help debugging.