stm32:can: help gcc recognise deliberate fall through

Very new gcc versions add a warning on switch cases that fall through.
While there's an option that accepts any comment as explaining it, it's
easier in our case to just use one of the "blessed" comments.  We can't
use the [[]] attributes for standards code, and the gcc specific
attributes are worse than the comments.  This has no functional change
whatsoever.

lib/stm32/can.c

@@ -406,28 +406,28 @@ int can_transmit(uint32_t canport, uint32_t id, bool ext, bool rtr,
 	switch (length) {
 	case 8:
 		tdhxr.data8[3] = data[7];
-		/* no break */
+		/* fall through */
 	case 7:
 		tdhxr.data8[2] = data[6];
-		/* no break */
+		/* fall through */
 	case 6:
 		tdhxr.data8[1] = data[5];
-		/* no break */
+		/* fall through */
 	case 5:
 		tdhxr.data8[0] = data[4];
-		/* no break */
+		/* fall through */
 	case 4:
 		tdlxr.data8[3] = data[3];
-		/* no break */
+		/* fall through */
 	case 3:
 		tdlxr.data8[2] = data[2];
-		/* no break */
+		/* fall through */
 	case 2:
 		tdlxr.data8[1] = data[1];
-		/* no break */
+		/* fall through */
 	case 1:
 		tdlxr.data8[0] = data[0];
-		/* no break */
+		/* fall through */
 	default:
 		break;
 	}