Bluetooth: Mesh: fix pb gatt fast advertising

pb gatt server starts fast advertising for the first 60 seconds.
After that, server goes into slow mode.
When pb gatt and pb adv work in parallel pd gatt changes fast to
slow modes after the first tx frame in pd adv (about 110ms)
since they both are handled in the same thread.
Actually, pb gatt never worked in fast mode
if pb adv was enabled (in most configuration cases).

Signed-off-by: Aleksandr Khromykh <Aleksandr.Khromykh@nordicsemi.no>