PCI: dwc: Rework MSI callbacks handler

Remove duplicate defines located on pcie-designware.h file already
available on /include/uapi/linux/pci-regs.h file.

Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>