powerpc/syscalls: Use sys_old_select() in ppc_select()

Instead of opencodying the copy of parameters, use
the generic sys_old_select().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4de983ad254739da1fe6e9f273baf387b7043ae0.1615398498.git.christophe.leroy@csgroup.eu