qtnfmac: handle channel switch events for connected stations only

Channel switch events from firmware should be processed only when STA
is already connected to BSS. On connect this notification is not needed
since full BSS info will be supplied by cfg80211_connect_result.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>