ppc: Don't update NIP in lswi/lswx/stswi/stswx

Instead, pass GETPC() result to the corresponding helpers. This
requires a bit of fiddling to get the PC (hopefully) right in
the case where we generate a program check, though the hacks there
are temporary, a subsequent patch will clean this all up by always
having the nip already set to the right instruction when taking
the fault.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[dwg: Fix trivial checkpatch warning]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>