wcn36xx: Track the band and channel we are tuned to

Track the band and channel we are currently tuned to by way of pointers to
the standard structures that describe them both embedded within the driver.

Tracking of the pair makes it much easier when implementing
ieee80211_ops->get_survey to return quickly captured metrics for the
currently tuned channel.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220115001646.3981501-3-bryan.odonoghue@linaro.org