pci: pci_mvebu: use dev_seq instead of static variable

pci: pci_mvebu: use dev_seq instead of static variable

PCI uclass maps PCI bus numbers to the seq member of struct udevice.
Use dev_seq(dev) as the bus number in mvebu_pcie_probe instead of an
incrementing a static variable.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Cc: Stefan Roese <sr@denx.de>
Cc: Phil Sutter <phil@nwl.cc>
Cc: Mario Six <mario.six@gdsys.cc>
Cc: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Stefan Roese <sr@denx.de>