Fix GCC7 warnings about switch-case fallthroughs

GCC7 with -Wextra warns about switch-case blocks which fallthrough with
"this statement may fall through [-Werror=implicit-fallthrough=]". This
can be fixed by adding "special" comments: "/* fallthrough */".

See https://gcc.gnu.org/gcc-7/changes.html

Change-Id: Iba0be791dbdd86984489b2d9a0592bb59828da1e
Signed-off-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-on: http://openocd.zylin.com/4174
Tested-by: jenkins

src/jtag/drivers/ftdi.c

@@ -855,6 +855,7 @@ COMMAND_HANDLER(ftdi_handle_set_signal_command)
 			ftdi_set_signal(sig, *CMD_ARGV[1]);
 			break;
 		}
+		/* fallthrough */
 	default:
 		LOG_ERROR("unknown signal level '%s', use 0, 1 or z", CMD_ARGV[1]);
 		return ERROR_COMMAND_SYNTAX_ERROR;

src/jtag/drivers/kitprog.c

@@ -657,6 +657,7 @@ static int kitprog_swd_switch_seq(enum swd_special_seq seq)
 				LOG_DEBUG("JTAG to SWD not supported");
 				/* Fall through to fix target reset issue */
 			}
+			/* fallthrough */
 		case LINE_RESET:
 			LOG_DEBUG("SWD line reset");
 			if (kitprog_swd_seq(SEQUENCE_LINE_RESET) != ERROR_OK)