cmd646: pass pci_dev as it needs it

Instead of doing tricks to get the pci_dev, just pass it in the 1st
place.  Patch is a bit longer that reverting the pci_dev field, but it
states more clearly (IMHO) what we are doing.

It also fixes the bm test, now that you told me that ->unit is not
always valid.

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