ddr: altera: sdram: Clean up sdram_write_verify()

ddr: altera: sdram: Clean up sdram_write_verify()

Clean the function up so that it's obvious what it is doing,
fix the formating strings in debug outputs, add kerneldoc.
Make the function return proper errno-compliant return values
and propagate this change throughout sdram.c

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