PCI: Use helper functions to access fields in struct msi_desc

Use helper functions to access fields in struct msi_desc, so we could
easily refine msi_desc later.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Reviewed-by: Yijing Wang <wangyijing@huawei.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Stuart Yoder <stuart.yoder@freescale.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Murali Karicheri <m-karicheri2@ti.com>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: Pratyush Anand <pratyush.anand@gmail.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Soeren Brinkmann <soren.brinkmann@xilinx.com>
Cc: Srikanth Thokala <sthokal@xilinx.com>
Cc: Rob Herring <robh@kernel.org>
Link: http://lkml.kernel.org/r/1436428847-8886-9-git-send-email-jiang.liu@linux.intel.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>