staging: wfx: ensure that received hif messages are never modified

There are no real reason to modify the data received from device. So,
let's mark the arguments constant.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191217161318.31402-23-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>