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