mtd: mxc-nand: Warn on unimplemented commands

The PARAM command was long unimplemented and it probably wasn't
noticed because chip probing using only the few bytes returned by the
READID command are good enough in most cases to determine the chip in
use.

Still to notice such a shortcoming earlier in the future would be nice
in case it's something more vital.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>