PCI: dwc: designware: Test PCIE_ATU_ENABLE bit specifically

The ATU CTRL2 register is 32 bits, and bits other than the enable bit may
be set.  To check whether the ATU is enabled or not, we should test the
enable bit specifically.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Joao Pinto <jpinto@synopsys.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>