powerpc/spufs: adjust list element pointer type

Other uses of &gang->aff_list_head, eg in spufs_assert_affinity, indicate
that the list elements have type spu_context, not spu as used here.  Change
the type of tmp accordingly.

This has no impact on the execution, because tmp is not used in the body of
the loop.

Fixes: c5fc8d2a ("[CELL] cell: add placement computation for scheduling of affinity contexts")
Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Reviewed-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1588929176-28527-1-git-send-email-Julia.Lawall@inria.fr