x86/vtd: Fix S3 resume following c/s 650c31d3af

x86/vtd: Fix S3 resume following c/s 650c31d3af

c/s 650c31d3af "x86/IRQ: fix locking around vector management" adjusted the
locking in adjust_irq_affinity().

The S3 path ends up here via iommu_resume() before interrupts are enabled, at
which point spin_lock_irq() fails ASSERT(local_irq_is_enabled()); but with no
working console.

Use spin_lock_irqsave() instead to cope with interrupts already being
disabled.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>