colibri_vf: Fix bstlen field

Commit 3f353cec ("vf610: refactor DDRMC code") changed the original
bstlen field from 3 to 0.

Restore the original value for proper behaviour.

Based on the patch from Anthony Felice <tony.felice@timesys.com>
for the vf610twr board.

Reported-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>