PCI: keystone: Cleanup ks_pcie_msi_irq_handler()

ks_pcie_msi_irq_handler() invokes ks_pcie_handle_msi_irq() for handling
the interrupts.

Having two functions for handling the interrupt was used when keystone
PCIe driver was implemented using two files but with commit b492aca3
("PCI: keystone: Merge pci-keystone-dw.c and pci-keystone.c"), which
merged the keystone PCIe driver to use a single file, two functions for
handling the interrupt handler are not required.

Handle MSI interrupt in a single interrupt handler here.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>