qtnfmac: use scan duration param for different scan types

Use scan duration param for both active and passive scan dwell times.
Document what different types of dwell times are used for. Explicitly
specify that if unset, automatic selection by device firmware
will be used.

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