powerpc/irq: Refactor irq_soft_mask_{set,or}_return()

This partialy reapply commit ef5b570d ("powerpc/irq: Don't
open code irq_soft_mask helpers") which was reverted by
commit 684c68d9 ("Revert "powerpc/irq: Don't open code
irq_soft_mask helpers"")

irq_soft_mask_set_return() and irq_soft_mask_or_return()
are overset of irq_soft_mask_set().

Have them use irq_soft_mask_set() instead of duplicating it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/18473da42362ee8f07bce36b9caef8ba77d7633f.1663656054.git.christophe.leroy@csgroup.eu