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>

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

@@ -42,6 +42,9 @@ unsigned long __init opal_get_boot_time(void)
 	__be64 __h_m_s_ms;
 	long rc = OPAL_BUSY;
 
+	if (!opal_check_token(OPAL_RTC_READ))
+		goto out;
+
 	while (rc == OPAL_BUSY || rc == OPAL_BUSY_EVENT) {
 		rc = opal_rtc_read(&__y_m_d, &__h_m_s_ms);
 		if (rc == OPAL_BUSY_EVENT)
@@ -49,16 +52,18 @@ unsigned long __init opal_get_boot_time(void)
 		else
 			mdelay(10);
 	}
-	if (rc != OPAL_SUCCESS) {
-		ppc_md.get_rtc_time = NULL;
-		ppc_md.set_rtc_time = NULL;
-		return 0;
-	}
+	if (rc != OPAL_SUCCESS)
+		goto out;
+
 	y_m_d = be32_to_cpu(__y_m_d);
 	h_m_s_ms = be64_to_cpu(__h_m_s_ms);
 	opal_to_tm(y_m_d, h_m_s_ms, &tm);
 	return mktime(tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
 		      tm.tm_hour, tm.tm_min, tm.tm_sec);
+out:
+	ppc_md.get_rtc_time = NULL;
+	ppc_md.set_rtc_time = NULL;
+	return 0;
 }
 
 void opal_get_rtc_time(struct rtc_time *tm)