iommu/amd: Fix error return code in irq_remapping_alloc()

Fix to return a negative error code from the alloc_irq_index() error
handling case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>