PCI: make msi_free_irqs() to use msix_mask_irq() instead of open coded write

Use msix_mask_irq() instead of direct use of writel, so as not to clear
preserved bits in the Vector Control register [31:1].

Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>