remoteproc: stm32: Fix error code in stm32_rproc_parse_dt()

There is a cut and paste bug so this code was returning the wrong
variable.  It should have been "ddata->hold_boot_rst" instead of
"ddata->rst".

Fixes: de598695 ("remoteproc: stm32: Allow hold boot management by the SCMI reset controller")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/6f457246-6446-42cb-81ae-d37221d726b1@kili.mountain


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>