[POWERPC] spufs: Make file-internal functions & variables static

There are a few symbols used only in one file within spufs; this change
makes them static where suitable.

Signed-off-by: Sebastian Siewior <sebastian@breakpoint.cc>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>