drivers/ddr/fsl: Fix typo in BIST test for DDR4

BIST test code has a typo, resulting the binding registers not
maintained as expected. This typo results BIST runs twice on
the covered memory.

Signed-off-by: York Sun <yorksun@freescale.com>
Reported-by: Shengzhou Liu <Shengzhou.Liu@freescale.com>