hw/ide: Extract IDEBus assignment into bmdma_init()

hw/ide: Extract IDEBus assignment into bmdma_init()

Every invocation of bmdma_init() is followed by `d->bmdma[i].bus = &d->bus[i]`.
Resolve this redundancy by extracting it into bmdma_init().

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20230531211043.41724-5-shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>