dmaengine: cv1800b-dmamux: fix device leak on route allocation

dmaengine: cv1800b-dmamux: fix device leak on route allocation

Make sure to drop the reference taken when looking up the DMA mux
platform device during route allocation.

Note that holding a reference to a device does not prevent its driver
data from going away so there is no point in keeping the reference.

Fixes: db7d07b5add4 ("dmaengine: add driver for Sophgo CV18XX/SG200X dmamux")
Cc: stable@vger.kernel.org	# 6.17
Cc: Inochi Amaoto <inochiama@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://patch.msgid.link/20251117161258.10679-5-johan@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>