hw/block/fdc-sysbus: Always mark sysbus floppy controllers as not having DMA

hw/block/fdc-sysbus: Always mark sysbus floppy controllers as not having DMA

The sysbus floppy controllers (devices sysbus-fdc and sun-fdtwo)
don't support DMA.  The core floppy controller code expects this to
be indicated by setting FDCtrl::dma_chann to -1.  This used to be
done in the device instance_init functions sysbus_fdc_initfn() and
sun4m_fdc_initfn(), but in commit 1430759ec3e we refactored this code
and accidentally lost the setting of dma_chann.

For sysbus-fdc this has no ill effects because we were redundantly
also setting dma_chann in fdctrl_init_sysbus(), but for sun-fdtwo
this means that guests which try to enable DMA on the floppy
controller will cause QEMU to crash because FDCtrl::dma is NULL.

Set dma_chann to -1 in the common instance init, and remove the
redundant code in fdctrl_init_sysbus() that is also setting it.

There is a six-year-old FIXME comment in the jazz board code to the
effect that in theory it should support doing DMA via a custom DMA
controller.  If anybody ever chooses to fix that they can do it by
adding support for setting both FDCtrl::dma_chann and FDCtrl::dma.
(A QOM link property 'dma-controller' on the sysbus device which can
be set to an instance of IsaDmaClass is probably the way to go.)

Fixes: 1430759ec3 ("hw/block/fdc: Extract SysBus floppy controllers to fdc-sysbus.c")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/958
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20220505101842.2757905-1-peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>