PCI/keystone: Use irq_data_get_msi_desc() to avoid redundant lookup of irq_data

Use irq_data_get_msi_desc() to avoid redundant lookup of irq_data while we
already have a pointer to corresponding irq_data.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Murali Karicheri <m-karicheri2@ti.com>
Cc: linux-pci@vger.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>