powerpc/powernv: Check OPAL RTC calls exists before using

Check that the OPAL_RTC_READ token exists before we use the OPAL RTC.

Refactors the code a little to merge error paths.

This avoids littering the OPAL console with:
  "OPAL: Called with bad token 3".

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>