dmaengine: stm32: dmamux: fix OF node leak on route allocation failure

dmaengine: stm32: dmamux: fix OF node leak on route allocation failure

Make sure to drop the reference taken to the DMA master OF node also on
late route allocation failures.

Fixes: df7e762db5f6 ("dmaengine: Add STM32 DMAMUX driver")
Cc: stable@vger.kernel.org      # 4.15
Cc: Pierre-Yves MORDRET <pierre-yves.mordret@foss.st.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Reviewed-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Link: https://patch.msgid.link/20251117161258.10679-12-johan@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>