iommu/vt-d: Mark functions as static in dmar.c

Mark the functions check_zero_address() and dmar_get_fault_reason() as
static in dmar.c because they are not used outside this file.

This eliminates the following warnings in dmar.c:
drivers/iommu/dmar.c:491:12: warning: no previous prototype for ‘check_zero_address’ [-Wmissing-prototypes]
drivers/iommu/dmar.c:1116:13: warning: no previous prototype for ‘dmar_get_fault_reason’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Joerg Roedel <joro@8bytes.org>