[POWERPC] Update MTFSF_L() comment

David Woodhouse points out that the comment accompanying the MTFSF_L
macro is misleading. We should make it clear that the L bit is ignored
on older CPUS, not the entire instruction.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>