[MTD] [NAND] Fix an off-by-one in a BUG_ON in CAFÉ ECC correction.

err_pos_lut[4096] of an array with 4096 elements is a bug.

Spotted by the Coverity checker.

While I was at it, I also converted it to ARRAY_SIZE().

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>