powerpc/powernv: Check OPAL RTC calls exists before using
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 <7b0ab45a730e48a69239010b5b8fe5fa4e8eaac6@neuling.org>
Signed-off-by: Michael Ellerman <864f124608374e06e4da1fa5d2e47ed839b95411@ellerman.id.au>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs