mbox_send_message is called by passing a local dummy message or
a function parameter. As the message is queued, it is dereferenced.
This works because the message field is not used by the stm32 ipcc
driver, but it is not clean.

Fix by passing a constant string in all cases.

The associated comments are removed because rproc should not have to
deal with the behavior of the mailbox frame.
Reported-by: NBjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: NArnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20210420091922.29429-1-arnaud.pouliquen@foss.st.comSigned-off-by: NBjorn Andersson <bjorn.andersson@linaro.org>