Correct a harmless problem when creating the DMA tag

used to map mbufs.  The maximum size should be MCLBYTES
and not SIS_TX_LIST_SZ.  This is probably a typo in
the original commit.

Tested by:	cognet

sys/pci/if_sis.c

@@ -1031,7 +1031,7 @@ sis_attach(dev)
 			BUS_SPACE_MAXADDR,	/* lowaddr */
 			BUS_SPACE_MAXADDR,	/* highaddr */
 			NULL, NULL,		/* filter, filterarg */
-			SIS_TX_LIST_SZ, 1,	/* maxsize,nsegments */
+			MCLBYTES, 1,		/* maxsize,nsegments */
 			BUS_SPACE_MAXSIZE_32BIT,/* maxsegsize */
 			0,			/* flags */
 			&sc->sis_tag);