tcl/interface/ftdi/sheevaplug: fix device description

Without this SheevaPlug debugging interface can't be matched.

Change-Id: Ifca149130d03c1aa165ed1123e8540e49485f023
Reported-by: Andreas Schneider <schneider.andi@gmail.com>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2265
Tested-by: jenkins