net: plip: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix multiple
warnings by explicitly adding multiple break statements instead of
letting the code fall through to the next case.

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

drivers/net/plip/plip.c

···
 		*data_p |= (c0 << 1) & 0xf0;
 		write_data (dev, 0x00); /* send ACK */
 		*ns_p = PLIP_NB_BEGIN;
+		break;
 	case PLIP_NB_2:
 		break;
 	}
···
 				return HS_TIMEOUT;
 			}
 		}
+		break;
 
 	case PLIP_PK_LENGTH_LSB:
 		if (plip_send(nibble_timeout, dev,