iommu/amd: Return proper error code in irq_remapping_alloc()

In the unlikely case when alloc_irq_table() is not able to return a
remap table then "ret" will be assigned with an error code. Later, the
code checks `index' and if it is negative (which it is because it is
initialized with `-1') and then then function properly aborts but
returns `-1' instead `-ENOMEM' what was intended.
In order to correct this, I assign -ENOMEM to index.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Joerg Roedel <jroedel@suse.de>