sparc/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 struct msi_desc later.

Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
Acked-by: David S. Miller <davem@davemloft.net>
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: Yijing Wang <wangyijing@huawei.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Eric Snowberg <eric.snowberg@oracle.com>
Link: http://lkml.kernel.org/r/1436428847-8886-8-git-send-email-jiang.liu@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>