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