isdn_v110: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/isdn/i4l/isdn_v110.c: In function ‘EncodeMatrix’:
drivers/isdn/i4l/isdn_v110.c:353:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (line >= mlen) {
       ^
drivers/isdn/i4l/isdn_v110.c:358:3: note: here
   case 128:
   ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

Notice that, in this particular case, the code comment is modified
in accordance with what GCC is expecting to find.

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/isdn/i4l/isdn_v110.c

@@ -354,7 +354,7 @@ EncodeMatrix(unsigned char *buf, int len, unsigned char *m, int mlen)
 				printk(KERN_WARNING "isdn_v110 (EncodeMatrix): buffer full!\n");
 				return line;
 			}
-			/* else: fall through */
+			/* fall through */
 		case 128:
 			m[line] = 128;	/* leftmost -> set byte to 1000000 */
 			mbit = 64;	/* current bit in the matrix line */