firmware: scmi: mailbox transport: fix probe failure implementation

Correct scmi mailbox probe function that can't free the scmi channel
instance since its auto-allocated by the device model framework.

Cc: Simon Glass <sjg@chromium.org>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>