[POWERPC] spufs: Fix restore_decr_wrapped() to match CBE Handbook

Based on an original patch from Masato Noguchi
<Masato.Noguchi@jp.sony.com>.

We're currently not restoring the SPE decrementer as specified by the
CBE handbook. This change fixes our implementation to match, and makes
the function read more like the docs.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>