i386: amd_iommu: fix MMIO register count and access

IOMMU MMIO registers are divided in two groups by their offsets.
Low offsets(<0x2000) registers are grouped into 'amdvi_mmio_low'
table and higher offsets(>=0x2000) registers are grouped into
'amdvi_mmio_high' table. No of registers in each table is given
by macro 'AMDVI_MMIO_REGS_LOW' and 'AMDVI_MMIO_REGS_HIGH' resp.
Values of these two macros were swapped, resulting in an OOB
access when reading 'amdvi_mmio_high' table. Correct these two
macros. Also read from 'amdvi_mmio_low' table for lower address.

Reported-by: Azureyang <azureyang@tencent.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
