jtag_vpi: correct runtest behavior

jtag_vpi: correct runtest behavior

Prevent the State Machine from moving at the end
of several cycles during run_test_idle
state.

Change-Id: Iabdf90a58334dd3d0e6e34b02983a85662062825
Signed-off-by: Megan Wachs <megan@sifive.com>
Reviewed-on: http://openocd.zylin.com/3635
Tested-by: jenkins
Reviewed-by: Jan Matyas <matyas@codasip.com>
Reviewed-by: Franck Jullien <franck.jullien@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>