ath5k: Fix double free on hw attach error path

If ath5k_hw_attach fails it will free sc->ah (local variable ah) before
returning. However, when it reports failure the caller (ath5k_pci_probe)
will also free sc->ah. Let the caller handle the deallocation, it does
so on further errors as well.

Signed-off-by: Jones Desougi <jones.desougi@27m.se>
Signed-off-by: John W. Linville <linville@tuxdriver.com>