powernv: Fix OPAL tracepoint code

Patch c49f6353 ("powernv: Add OPAL tracepoints") has a spurious
store to the stack:

	ld      r12,opal_tracepoint_refcount@toc(r2);           \
	std     r12,32(r1);                                     \

The store was originally used to save the current tracepoint status
so the entry and the exit tracepoints were always balanced. In the
end I just created a separate path when tracepoints are enabled.

The offset on the stack used for this store is not valid for ABIv2
and it causes strange issues. I noticed it because OPAL console input
was broken.

Fixes: c49f6353

 ("powernv: Add OPAL tracepoints")
Cc: <stable@vger.kernel.org> # v3.17+
Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>