pci: Skip configuring invalid P2P bridge devices

Function dm_pci_hose_probe_bus() expects that bus is valid PCI device with
Bridge header type (0x01). So add check before touching PCI config space to
prevent misconfiguring some non-standard device.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>