Subpanel Vs. 220v-240v NEMA 14-50
Breakers protect WIRE and #8 conductors allow you to use a 40A breaker in the Main panel to feed the sub-panel. That's your only limiting factor here. It makes no difference what the PANEL is rated for so long as it is more than adequate for a 40A circuit, which is is. So get off of that side track.

In the sub panel, you can put whatever else you want in there that is 40A or less. But if your NEMA 14-50 outlet has something plugged into it that is drawing 40A, and you use something else from that sub-panel that pulls any more, the breaker in the Main panel will trip to protect those conductors, so be aware of what you use in that garage. 

The tricky part is that a 14-50 outlet is good for 50A, not 40 A, so if you needed that receptacle (i.e. a Tesla charger), ostensibly it means it likely could draw more than 40A which would be a problem for your circuit. If you really needed a 50A circuit, you needed #6 conductors all around.

Edit: Sorry, I was thinking building wire like NM, which is used from the 60C column, you said you have THHN in conduit, so it is good for 50A, not 40A. So you can use a 50A breaker in the Main panel.