ddr: altera: Repair DQ window centering code

The code uses a lot of signed numbers, which ended up in variables
of unsigned type, which resulted in all sorts of underflows. This
in turn caused incorrect calibration on certain boards. Moreover,
repair the readout of the DQ delay, which was being pulled from
wrong register.

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