PCI: dra7xx: Enable MSI and legacy interrupts simultaneously

pci-dra7xx driver had a bug in that if CONFIG_PCI_MSI config is enabled, it
doesn't support legacy interrupt.  Fix it here so that both MSI and legacy
interrupts can be enabled simultaneously and the interrupt mechanism
supported by the endpoint device will be used.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>