powerpc/lib/sstep: use truncate_if_32bit()

Use truncate_if_32bit() when possible instead of open coding.

truncate_if_32bit() returns an unsigned long, so don't use it when
a signed value is expected.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/7e1c07123f13156d4a27991a2e2694fb584bc068.1642752375.git.christophe.leroy@csgroup.eu