jtag/drivers: trivial variable renaming

Rename ublast_sleep() time variable from ms to us as the
unit is in micro-seconds.

Change-Id: I07ff9c36d302d5d59edb8a5faa73d78cdd6145a4
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Reviewed-on: http://openocd.zylin.com/1457
Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>