ECC calculation bug in nand_write_page()

[Alex Samoutin]

I found that ECC calculation is worng when we do partial write to sector.

When starting byte !=0 and eccmode == NAND_ECC_SOFT this function made

/* Read back previous written data, if col > 0 */

for correct ECC calculation. It is ok. Hovewer  if col==0 but last !=0 it
would do the same (read back previos written data) for the
tail of block. But it didn't.

Alex