mx6: ddr: Fix disabling on-die termination

In case rtt_nom is set to 0 keep ODT disabled (MMDC MPODTCTRL = 0).
No changes required for DDR MR1 Rtt_Nom impedance register, 0 value is
already handled correctly.

No board is currently affected by this change (rtt_nom != 0 on all i.MX6
ddr3 boards), this will be used by a follow-up change.

Fixes: fe0f7f78 ("mx6: add mmdc configuration for MX6Q/MX6DL")
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Acked-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>