drivers/ftdi: warn the user if he tries to use undefined reset signals

It is really an error to try to use a reset_config that doesn't match
adapter's capabilities, however OpenOCD has no way to specify them.

Using wrong reset_config might lead to very confusing behaviour, so I
think LOG_ERROR is justified here.

Change-Id: I1c6dcfa7c0d78829229a850189cad646b565dd66
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1948
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/jtag/drivers/ftdi.c

@@ -472,8 +472,11 @@ static void ftdi_execute_reset(struct jtag_command *cmd)
 		tap_set_state(TAP_RESET);
 
 	struct signal *trst = find_signal_by_name("nTRST");
-	if (trst && cmd->cmd.reset->trst == 1) {
-		ftdi_set_signal(trst, '0');
+	if (cmd->cmd.reset->trst == 1) {
+		if (trst)
+			ftdi_set_signal(trst, '0');
+		else
+			LOG_ERROR("Can't assert TRST: nTRST signal is not defined");
 	} else if (trst && cmd->cmd.reset->trst == 0) {
 		if (jtag_get_reset_config() & RESET_TRST_OPEN_DRAIN)
 			ftdi_set_signal(trst, 'z');
@@ -482,8 +485,11 @@ static void ftdi_execute_reset(struct jtag_command *cmd)
 	}
 
 	struct signal *srst = find_signal_by_name("nSRST");
-	if (srst && cmd->cmd.reset->srst == 1) {
-		ftdi_set_signal(srst, '0');
+	if (cmd->cmd.reset->srst == 1) {
+		if (srst)
+			ftdi_set_signal(srst, '0');
+		else
+			LOG_ERROR("Can't assert SRST: nSRST signal is not defined");
 	} else if (srst && cmd->cmd.reset->srst == 0) {
 		if (jtag_get_reset_config() & RESET_SRST_PUSH_PULL)
 			ftdi_set_signal(srst, '1');