powerpc/pci: remove duplicate declaration of pci_bus_find_capability

pci_bus_find_capability() is decleared in pci.h, so it is not necessary to do
it again.

This patch removes it.

Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>