[MTD] [NAND] Don't panic if a controller driver does ecc its own way.

Some hardware, such as the enhanced local bus controller used on some
mpc83xx chips, does ecc transparently when reading and writing data, rather
than providing a generic calculate/correct mechanism that can be exported to
the nand subsystem.

The subsystem should not BUG() when calculate, correct, or hwctl are
missing, if the methods that call them have been overridden.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>