[linda] Wait up to 20us for link state to update

Some subnet managers expect the GetResponse from a SetPortInfo MAD to
contain the new link state.  The transition is not immediate, so we
often end up returning the previous link state.  This can cause the SM
to fail to activate the port.

Fix by waiting for up to 20us for the link state transition to take
effect.

src/drivers/infiniband/linda.h

@@ -270,4 +270,7 @@ enum linda_link_state {
 	LINDA_LINK_STATE_ACT_DEFER = 4,
 };
 
+/** Maximum time to wait for link state changes, in us */
+#define LINDA_LINK_STATE_MAX_WAIT_US 20
+
 #endif /* _LINDA_H */