jtag: drivers: ulink: fix uninitialised variable warning

This prevents clean build when --enable-verbose-jtag-io is used.

Change-Id: I5c9e6968cfa425b1f6f92f59156b6ae38cb9af18
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/3907


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>