PCI: dwc: dra7xx: Use RW1C for IRQSTATUS_MSI and IRQSTATUS_MAIN

Previously, we tried to clear interrupt requests by clearing bits in the
PCIECTRL_DRA7XX_CONF_IRQSTATUS_MSI and PCIECTRL_DRA7XX_CONF_IRQSTATUS_MAIN
registers.  But per the TRM, these fields are RW1C, so we must *set* bits
to clear the interrupt bits.

Fixes: 47ff3de9 ("PCI: dra7xx: Add TI DRA7xx PCIe driver")
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
[bhelgaas: changelog]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>