remoteproc: mediatek: Fix error code in scp_rproc_init()

Set the error code to ERR_PTR(-ENOMEM).  Otherwise if there is an
allocation failure it leads to a NULL dereference in the caller.

Fixes: c08a8249

 ("remoteproc: mediatek: Support setting DRAM and IPI shared buffer sizes")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/b2114e3c-fa64-4edb-a1ff-d2009e544c3f@moroto.mountain
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>