iommu/amd: Fix logic to determine and checking max PASID
In reality, the spec can only support 16-bit PASID since
INVALIDATE_IOTLB_PAGES and COMPLETE_PPR_REQUEST commands only allow 16-bit
PASID. So, we updated the PASID_MASK accordingly and invoke BUG_ON
if the hardware is reporting PASmax more than 16-bit.
Besides, max PASID is defined as ((2^(PASmax+1)) - 1). The current does not
determine this correctly.