Fix strange RTL behavior

This commits have fixed #1441 for me. Please review and cross test.

The behaviour should now be:

For COM_DL_LOSS_EN ==0:

- Mission finished, no RC, datalink ok: AUTO_RCRECOVER (RTL)
- Mission finished, no RC, datalink lost: AUTO_RCRECOVER (RTL)

For COM_DL_LOSS_EN == 1:

- Mission finished, no RC, datalink ok: AUTO_MISSION (loitering)
- Mission finished, no RC, datalink lost: AUTO_RCRECOVER (RTL)

I think the check for mission_finished is missing here. On the other hand, this now does the same as the block starting at line 552 https://github.com/PX4/Firmware/pull/1443/files#diff-20f2c9d76df9330422532b87ab964f86R552
(was probably wrong/double before your changes)
If `data_link_loss_enabled` is true and we are in auto we can go into failsafe anyway as soon as datalink is lost (regardless if the mission is finished or not). So if I am not mistaken we can remove this block here and just keep the one at 552 as long as the same nav_state is set.