Manually adjust flash power on Canon Speedlite 90EX?

So I bought an EOS M and it came with the (seemingly) useless Speedlite 90EX. While it does TTL on the EOS M, my main system is around a Nikon D5200. I'm thinking about getting a new manual flash (TTL is too expensive for me) to use off camera with some cheapo transmitters and receivers off eBay. Is there any way to control the flash power on this or will I have to buy a new manual flash (like the Yongnuo)?

Canon Speedlite are designed around Canon cameras. You can probably control the flash manually through the camera menus. That doesn't help when you want to use it with a Nikon off camera though.

The easiest way to control flash if you can't figure out the controls is to play with distance. The nearer the flash is to the subject, the stronger the light will be.