powerpc/powernv: only call OPAL_RESEND_DUMP if firmware supports it

Not all OPAL platforms support resending system dumps, so check
that current firmware supports it first. Otherwise we get firmware
complaining:
"OPAL: Called with bad token 91 !"

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