PCI MSI: Unmask MSI if setup failed

The initial state of mask register of MSI is unmasked.  We set it
masked before calling arch_setup_msi_irqs().  If arch_setup_msi_irq()
fails, it is better to restore the state of the mask register.

Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>