qtnfmac: do not cache current channel info in driver's state

Linux Wireless device structure already has current channel
information that can be used when needed. Start using it.
Since driver's channel info is not used anymore, remove it.

Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>