powerpc/irq: Simplify __do_irq()

Remove duplicated code by implementing a proper if/else.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/5a3b21311191f1240850db6ab29b19ac7885fe03.1654769775.git.christophe.leroy@csgroup.eu