powerpc/interrupt: Remove prep_irq_for_user_exit()

prep_irq_for_user_exit() has only one caller, squash it
inside that caller.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210617155116.2167984-18-npiggin@gmail.com