They don't just magically adapt because they are being used. To adapt they would have to be attacked by bacteria, and only the stronger ones survive, but MRSA doesn't attack onions.
They could adapt because humans only grow the ones that are the better of the batch at doing what we want them to do. I can't see it working as fast as bacteria, but it could be another option.
It seems like it should be at pretty much the same speed, if an equal size amount of garlic plants were being attacked by MRSA. Doesn't matter if natural selection of human-guided selection does the culling.
I suppose it might be expensive to grow lots and lots of garlic plants and test them all for efficacy, though.
A generation of bacteria is way shorter lived than a generation of garlic. It would depend on how much the garlic was used though I'm pretty sure, though you might need to talk to an actual biologist for details.
Also, unless garlic defended itself from MRSA by killing it human guided selection would be better, what if the garlic is sneaky and figures out another way to survive that doesn't help humans.