remoteproc: stm32: Improve crash recovery time

When a stop is requested on a crash, it is useless to try to shutdown it
gracefully, it is crashed.

In this case don't send the STM32_MBX_SHUTDOWN mailbox message that
will block the recovery during 500 ms, waiting an answer from the
coprocessor.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20211221143129.18415-1-arnaud.pouliquen@foss.st.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>