ddr: altera: Clean up set_rank_and_odt_mask() part 1

ddr: altera: Clean up set_rank_and_odt_mask() part 1

First, invert the logic of the if (odt_mode == ...) conditional to make
the OFF mode harder to miss. It is a short piece of code right at the
end, so move it up.

Also, clean up data types and constify where applicable and clean up
the cs_and_odt_mask assignment. No functional change.

Signed-off-by: Marek Vasut <marex@denx.de>