mtd: atmel_nand: fix pmecc selction for ecc requirement typo

Atmel PMECC support 2, 4, 8, 12, 24 bit error correction.
So if the ecc requirement in ONFI is <= 2, 4, 8, 12, 24.
We will use 2, 4, 8, 12, 24.

This patch fix the typo. Use '<=' replace '<'.

Reported-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>