ocxl: afu_irq only deals with IRQ IDs, not offsets

The use of offsets is required only in the frontend, so alter
the IRQ API to only work with IRQ IDs in the backend.

Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
Acked-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>