Assert that invalid bus widths can't be passed to bus_width_str().

This appeases a -Wreturn-type warning from GCC.

Reviewed by:	imp
Differential Revision:	https://reviews.freebsd.org/D31935

sys/cam/mmc/mmc_da.c

@@ -1178,6 +1178,8 @@ static inline const char
 		return ("4-bit");
 	case bus_width_8:
 		return ("8-bit");
+	default:
+		__assert_unreachable();
 	}
 }