steppenprobe: fix file permission

Commit 895d4a5 ("tcl/interface/ftdi: Add Steppenprobe open
hardware interface") erroneously set the execution permission to
the configuration file.

Strip the execution permission.

Change-Id: I556451d5e6fee4aee385451e8c90216a25b6ef46
Signed-off-by: Antonio Borneo <[email protected]>
Fixes: http://openocd.zylin.com/5653
Reviewed-on: http://openocd.zylin.com/6038
Reviewed-by: Paul Fertser <[email protected]>
Tested-by: Paul Fertser <[email protected]>

Author: borneoa