ddr: altera: Tweak DQS tracking enable handling

In the most unlikely case the DQS tracking was to be disabled,
make sure we do not errornously re-enable it. Note that DQS
tracking is enabled on all systems observed thus far.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Chin Liang See <clsee@altera.com>