Do not try to cache current operational channel info in driver, this
is a potential source of synchronization issues + driver does not
really need that info.

Introduce GET_CHANNEL command and process it appropriately.
Signed-off-by: NIgor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>