staging: mt7621-pci: remove remaining pci_legacy dependant code

pcibios_* remaining code is not neccessary at all. We can use
map_irq set to of_irq_parse_and_map_pci driver 'probe' function.
Remove this code.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Tested-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>