ath5k: fix memory leak when fewer than N_PD_CURVES are in use

We would free the proper number of curves, but in the wrong
slots, due to a missing level of indirection through
the pdgain_idx table.

It's simpler just to try to free all four slots, so do that.

Cc: stable@kernel.org
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>