mtd: atmel_nand: move the sanity check to the beginning of pmecc_enable()

It is better to do the sanity check for the parameter before any hardware
operation.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>