powerpc/interrupt: Fix CONFIG ifdef typo

CONFIG_PPC_BOOK3S should be CONFIG_PPC_BOOK3S_64. restore_math is a
no-op for other configurations.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[np: split from another patch]
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210617155116.2167984-2-npiggin@gmail.com