powerpc/powernv: Update firmware archaeology around OPAL_HANDLE_HMI

The first machines to ship with OPAL firmware all got firmware updates
that have the new call, but just in case someone is foolish enough to
believe the first 4 months of firmware is the best, we keep this code
around.

Comment is updated to not refer to late 2014 as recent or the future.

Signed-off-by: Stewart Smith <stewart@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>