vf610twr: Fix typo in DRAM init

This commit fixes a typo in vf610twr DRAM init that was causing a hang in
U-Boot for the Vybrid Tower. This typo was introduced in commit 3f353cec
(vf610: refactor DDRMC code).

Signed-off-by: Anthony Felice <tony.felice@timesys.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>