treewide: Use helper function to access irq_data->msi_desc

Use irq_data access helper to access irq_data->msi_desc, so we can
move msi_desc from struct irq_data into struct irq_common_data later.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>