remoteproc: st: Fix error return code in st_rproc_probe()

Fix to return a negative error code from the st_rproc_state() error
handling case instead of 0, as done elsewhere in this function.

Fixes: 63edb031 ("remoteproc: Supply controller driver for ST's Remote Processors")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>