clock: offer a method to get the first port in the list.

This function will allow the TC code to iterate over the ports without
calling into the clock logic.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>

clock.c

@@ -1066,6 +1066,11 @@ UInteger8 clock_domain_number(struct clock *c)
 	return c->dds.domainNumber;
 }
 
+struct port *clock_first_port(struct clock *c)
+{
+	return LIST_FIRST(&c->ports);
+}
+
 void clock_follow_up_info(struct clock *c, struct follow_up_info_tlv *f)
 {
 	c->status.cumulativeScaledRateOffset = f->cumulativeScaledRateOffset;

clock.h

@@ -103,6 +103,13 @@ void clock_destroy(struct clock *c);
  */
 UInteger8 clock_domain_number(struct clock *c);
 
+/**
+ * Obtains a reference to the first port in the clock's list.
+ * @param c  The clock instance.
+ * @return   A pointer to a port, or NULL if no ports are present.
+ */
+struct port *clock_first_port(struct clock *c);
+
 /**
  * Provide the follow_up info TLV from a slave port.
  * @param c  The clock instance.