mwl: return ENOMEM when rx buffer allocation fails

The malloc() failure path returned error, which is 0 at this point,
so callers would treat the allocation failure as success.
Return ENOMEM instead to correctly propagate the out-of-memory condition.

Reviewed by:	adrian
Differential Revision:	https://reviews.freebsd.org/D42282

sys/dev/mwl/if_mwl.c

@@ -2165,7 +2165,7 @@ mwl_rxdma_setup(struct mwl_softc *sc)
 	bf = malloc(bsize, M_MWLDEV, M_NOWAIT | M_ZERO);
 	if (bf == NULL) {
 		device_printf(sc->sc_dev, "malloc of %u rx buffers failed\n", bsize);
-		return error;
+		return ENOMEM;
 	}
 	sc->sc_rxdma.dd_bufptr = bf;