ath5k: add bounds check to pdadc table

We check the bounds on pdadc once when correcting for
negative curves but not when we later copy values from
from the pdadc_tmp array, leading to a potential overrun.

Although we shouldn't hit this case in practice, let's
be consistent.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>