ath5k: Add proper dependency for ATH5K_AHB

The CONFIG_ATH5K_AHB could be enabled on ATH25 system without enabling
ATH5K driver itself.  This does not make sense because CONFIG_ATH5K_AHB
controls object build within drivers/net/wireless/ath/ath5k/ so enabling
it without CONFIG_ATH5K brings nothing.

Add proper dependency to CONFIG_ATH5K_AHB.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>