mv_ddr: ddr3: only use active chip-selects when tuning ODT

Inactive chip-selects will give invalid values for read_sample so don't
consider them when trying to determine the overall min/max read sample.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>

[https://github.com/MarvellEmbeddedProcessors/mv-ddr-marvell/pull/18

]
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>