PCI: xilinx: Rename xilinx_pcie_link_is_up() to xilinx_pcie_link_up()

Rename xilinx_pcie_link_is_up() to xilinx_pcie_link_up() to follow the
convention of other drivers.  No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>