AMD IOMMU: cover all functions of a device even if ACPI only tells us of func 0

AMD IOMMU: cover all functions of a device even if ACPI only tells us of func 0

This ought to work as all functions of a device have the same place in
the bus topology, i.e. use the same IOMMU.

Also fix the type of ivrs_bdf_entries (when it's 'unsigned short' and
the last device found on a segment is ff:1f.x, it would otherwise end
up being zero).

And drop the bogus 'last_bdf' static variable, which conflicted anyway
with various functions' parameters.

Signed-off-by: Jan Beulich <jbeulich@suse.com>