Check that the OPAL_DUMP_READ token exists before initalising the elog
infrastructure.

This avoids littering the OPAL console with:
  "OPAL: Called with bad token 91"
Signed-off-by: NMichael Neuling <mikey@neuling.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>