ddr: altera: sdram: Clean up compute_errata_rows() part 2

ddr: altera: sdram: Clean up compute_errata_rows() part 2

Remove all parameters of this function, since they are only constants
passed in from another function, so make them local. Also, rename the
function to get_errata_rows() as this is closer to what it does.

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