powerpc/rtas_flash: Update return token comments

Add proper comment to ibm,validate-flash-image RTAS call
update result tokens.

Note: Only comment section is modified, no code change.

Signed-off-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>