workaround for cmd646 bmdma register access while no dma is active

This is a workaround only, and is a partial revert
of a few changes to BMDMAState which removed pci_dev
field on the way.

- cmd646 pci_from_bm() expects bm->unit value to
correspond with bm data being passed to callback
as opaque pointer. This breaks when write to dma
control register of second channel happens when no
dma operation is in progress, so bm->unit is zero
for second channel, and pci_from_bm() returns garbage
pointer. Crash happens shortly after that while
dereferencing that pointer.

v0->v1: cleaned up dead code from pci_from_bm.

Signed-off-by: Igor V. Kovalenko <igor.v.kovalenko@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
