target/imx6: Fix typo in setting _SJC_TAPID from SJC_TAPID variable

The set command was missing the $ prefix on the SJC_TAPID variable
and so would fail if SJC_TAPID was set

Change-Id: Ib9af58f5188bd8a2bc3f888309f203d624476c27
Signed-off-by: Alex Murray <alex.murray@cohdawireless.com>
Reviewed-on: http://openocd.zylin.com/1811
Tested-by: jenkins
Reviewed-by: Jens Bauer <jens@gpio.dk>
Reviewed-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

tcl/target/imx6.cfg

@@ -21,7 +21,7 @@ jtag newtap $_CHIPNAME sdma -irlen 4 -ircapture 0x00 -irmask 0x0f
 
 # System JTAG Controller
 if { [info exists SJC_TAPID] } {
-        set _SJC_TAPID SJC_TAPID
+        set _SJC_TAPID $SJC_TAPID
 } else {
         set _SJC_TAPID 0x0191c01d
 }