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

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>