[MTD] NAND Add read/write function pointers to struct nand_ecc_ctrl

Add read/write function pointers to struct nand_ecc_ctrl to
prepare the modulaization of nand_read/write functions. The
current implementation handles every type of ecc mode
software/hardware and all kinds of strange ecc placement
schemes in one switch/if construct. Thats too complex to
maintain and too inflexible to expand. Modularization will
also shorten the code pathes of the read/write functions.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>