Bluetooth: Mesh: Check avail conn before restarting pb-gatt srv adv

When a provisioner connects to the device, gatt_connected callback is
called in pb_gatt_srv.c, then pb-gatt advertising is stopped and adv_sent
callback is triggered in adv_ext.c. adv_sent callback reschedules
advertising and eventually calls bt_mesh_adv_gatt_send. Since the device
is not provisioned yet, it tries to start pb-gatt advertising again. If
number of available connections is reached, this results in "Advertising
failed" error in adv_ext.c.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>