microblaze/PCI: Remove unnecessary struct pci_dev declaration

All users of arch/microblaze/include/asm/pci.h get it by including
include/linux/pci.h, which in turn includes <asm/pci.h> after it declares
struct pci_dev.

The forward declaration here is unnecessary, so remove it.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Michal Simek <monstr@monstr.eu>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>