Hmm...i'm not sure this will work, but try this:
1.) Make sure there is no disc in your system.
2.) Try downloading official firmware 3.56 again and put the PS3UPDAT.PUP in this directory on the root of your stick (Create 2 folders that are just like this): PS3/UPDATE/PS3UPDAT.PUP
3.) Hopefully, that will fix the issue. It might be that your firmware was corrupted or something.
Hope it works!