ocxl: Clarify error path in setup_xsl_irq()

Implementing rollback with goto and labels is a common practice that
leads to prettier and more maintainable code. FWIW, this design pattern
is already being used in alloc_link() a few lines below in this file.

Do the same in setup_xsl_irq().

Signed-off-by: Greg Kurz <groug@kaod.org>
Acked-by: Frederic Barrat <fbarrat@linux.ibm.com>
Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>