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>

arch/powerpc/platforms/powernv/opal-rtc.c

@@ -50,7 +50,7 @@ unsigned long __init opal_get_boot_time(void)
 		rc = opal_rtc_read(&__y_m_d, &__h_m_s_ms);
 		if (rc == OPAL_BUSY_EVENT)
 			opal_poll_events(NULL);
-		else
+		else if (rc == OPAL_BUSY)
 			mdelay(10);
 	}
 	if (rc != OPAL_SUCCESS)