powerpc/xive: Simplify xive_do_source_eoi()

Previous patches removed the need of the first argument which was a
hack for Firwmware EOI. Remove it and flatten the routine which has
became simpler.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201210171450.1933725-12-clg@kaod.org