jtag/startup.tcl: fix regression with autoselecting JTAG

This regression was introduced with d90b86d8. "transport select" doesn't
throw an error anymore and autoselects the first available transport on
its own.

Reported by moyix on IRC.

Change-Id: I3f303c0372e915931cca4b28af450694abc1a63e
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2693
Tested-by: jenkins

src/jtag/startup.tcl

@@ -89,9 +89,8 @@ proc measure_clk {} {
 add_help_text measure_clk "Runs a test to measure the JTAG clk. Useful with RCLK / RTCK."
 
 proc default_to_jtag { f args } {
-	if [catch {transport select} current_transport] {
-		echo "Info : session transport was not selected, defaulting to JTAG"
-		transport select jtag
+	set current_transport [transport select]
+	if {[using_jtag]} {
 		eval $f $args
 	} {
 		error "session transport is \"$current_transport\" but your config requires JTAG"