ath5k: free sc->ah in ath5k_deinit_softc

Nothing else is freeing it, and this mirrors the error handling path
already in ath5k_init_softc.

https://bugzilla.kernel.org/show_bug.cgi?id=37592



Signed-off-by: John W. Linville <linville@tuxdriver.com>
Reported-by: Tsyvarev Andrey <tsyvarev@ispras.ru>