net: dsa: mv88e6xxx: add default case to switch

Add default case to switch in order to avoid any chance of using an
uninitialized variable _low_, in case s->type does not match any of
the listed case values.

Addresses-Coverity-ID: 1398130
Suggested-by: Andrew Lunn <[e-mail]>
Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/dsa/mv88e6xxx/chip.c

···
 		mv88e6xxx_g1_stats_read(chip, reg, &low);
 		if (s->sizeof_stat == 8)
 			mv88e6xxx_g1_stats_read(chip, reg + 1, &high);
+		break;
+	default:
+		return UINT64_MAX;
 	}
 	value = (((u64)high) << 16) | low;
 	return value;