powerpc/powernv: Only delay opal_rtc_read() retry when necessary

Only delay opal_rtc_read() when busy and are going to retry.

This has the advantage of possibly saving a massive 10ms off booting!

Kudos to Stewart for noticing.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Reviewed-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>