usb: mtu3: unlock @mtu->lock just before giving back request

No need unlock @mtu->lock when unmap request, unlock it just before
giving back request, due to we do not lock this spinlock when map
the request.

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20230417025203.18097-4-chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>