pcie/layerscape: fix bug in bus number computation when setting msi-map

pcie/layerscape: fix bug in bus number computation when setting msi-map

When multiple PCI cards are present in an ls2080a board, the second
card does not get its msi-map set up properly due to a bug in
computing the bus number.

The bus number returned by PCI_BDF() is not the actual PCI bus
number, but instead represents a global u-boot PCI bus number. A
given bus number is relative to hose->first_busno, so that has to be
subtracted from the PCI device id.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@nxp.com>
Acked-by: Stuart Yoder <stuart.yoder@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>