nds32: Avoid detected JTAG clock

nds32: Avoid detected JTAG clock

AICE2 doesn't support scan for the maximum clock frequency of
JTAG chain. It will cause USB command timeout.

Change-Id: I41d1e3be387b6ed5a4dd0be663385a5f053fbcf9
Signed-off-by: Hellosun Wu <wujiheng.tw@gmail.com>
Reviewed-on: http://openocd.zylin.com/4292
Tested-by: jenkins
Reviewed-by: Hsiangkai Wang <hsiangkai@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>