[MTD] NAND: nand_write_ecc memory and OOB corruption

Nathan Roberts noticed the nand_write_ecc index into oobbuf goes out of
bounds when crossing an erase block boundary, causing incorrect OOB data
to be written and corrupting memory.  Reset the index to zero after
re-preparing oobbuf for a new erase block.

Signed-off-by: Todd Poynor <tpoynor@mvista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>