[infiniband] Remove the return status code from MAD handlers

MAD handlers have to set the status fields within the MAD itself
anyway, in order to provide a meaningful response MAD; the additional
gPXE return status code is just noise.

Note that we probably don't need to ever explicitly set the status to
IB_MGMT_STATUS_OK, since it should already have this value from the
request.  (By not explicitly setting the status in this way, we can
safely have ib_sma_set_xxx() call ib_sma_get_xxx() in order to
generate the GetResponse MAD without worrying that ib_sma_get_xxx()
will clear any error status set by ib_sma_set_xxx().)

src/include/gpxe/ib_gma.h

@@ -34,13 +34,12 @@ struct ib_gma_handler {
 	 *
 	 * @v gma	General management agent
 	 * @v mad	MAD
-	 * @ret rc	Return status code
 	 *
 	 * The handler should modify the MAD as applicable.  If the
 	 * handler returns with a non-zero value in the MAD's @c
 	 * method field, it will be sent as a response.
 	 */
-	int ( * handle ) ( struct ib_gma *gma, union ib_mad *mad );
+	void ( * handle ) ( struct ib_gma *gma, union ib_mad *mad );
 };
 
 /** GMA attribute handlers */