net/if: Fix documentation comment marker.

Documentation comments should begin with /**

Change-Id: I59867e8aad340dac4d66f86e09f4f8ae9d3d75fb
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>

include/net/net_if.h

diff --git a/include/net/net_if.h b/include/net/net_if.h
index 3b1a066..d76477d 100644
--- a/include/net/net_if.h
+++ b/include/net/net_if.h
@@ -1117,7 +1117,7 @@
  */
 int net_if_up(struct net_if *iface);
 
-/*
+/**
  * @brief Bring interface down
  *
  * @param iface Pointer to network interface