tcl: [2/3] prepare for jimtcl 0.81 'expr' syntax change

tcl: [2/3] prepare for jimtcl 0.81 'expr' syntax change

Jimtcl commit 1843b79a03dd ("expr: TIP 526, only support a single
arg") drops the support for multi-argument syntax for the TCL
command 'expr'.

Enclose within double quote the argument of 'expr' when there is
the need to concatenate strings.

Change-Id: Ic0ea990ed37337a7e6c3a99670583685b570b8b1
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/6160
Tested-by: jenkins