dm: pci: Correct primary/secondary/subordinate bus number assignment

In driver model, each pci bridge device has its own hose structure.
hose->first_busno points to the bridge device's device number, so
we should not substract hose->first_busno before programming the
bridge device's primary/secondary/subordinate bus number registers.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>