Bluetooth: mesh: Account for scan window delaying adv events

New advertising started while scanning is already enabled
would delay the first advertisement event until the end of
the current overlapping scan window in the Zephyr native BLE
controller implementation. Hence, consider this scan window
duration when calculating the advertising stop.

Relates to: #6083

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>