atm: fore200e: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a fallthrough pseudo-keyword.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <[e-mail]>

drivers/atm/fore200e.c

···
 	/* XXX shouldn't we *start* by deregistering the device? */
 	atm_dev_deregister(fore200e->atm_dev);
 
+	fallthrough;
     case FORE200E_STATE_BLANK:
 	/* nothing to do for that state */
 	break;