colibri_imx6: fix RALAT and WALAT values

Running a memtest in U-Boot and Linux shows that some Colibri iMX6
produce bitflips at temperatures above 60°C. This happens because the
RALAT and WALAT values on the Colibri iMX6 are too low. The problems
were introduced by commit 09dbac81 ("mx6: ddr: Restore ralat/walat
in write level calibration") before the calibration process overwrote
the values and set them to the maximum value. With this commit, we make
sure that the RALAT and WALAT values are set to the maximum values
again. This has been proven to work for years.

Fixes: 09dbac81 ("mx6: ddr: Restore ralat/walat in write level calibration")
Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>