mv_ddr: ddr3: Use correct bitmask for read sample delay

mv_ddr: ddr3: Use correct bitmask for read sample delay

In the Armada 385 functional spec (MV-S109094-00 Rev. C) the read sample
delay fields are 5 bits wide. Use the correct bitmask of 0x1f when
extracting the value.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
[upstream https://github.com/MarvellEmbeddedProcessors/mv-ddr-marvell/pull/22]
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>