iommu/vt-d: Fix signedness bug in alloc_irte()

"index" needs to be signed for the error handling to work.  I deleted a
little bit of obsolete cruft related to "index" and "start_index" as
well.

Fixes: 360eb3c5 ('iommu/vt-d: use dedicated bitmap to track remapping entry allocation status')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>