I recently completed the quest "I fought the law", with the ending that favored the powder gangers. I thought that after I received the orders to attack the NCRCF I could convince the powder gangers to either surrender, or to leave the facility, not realizing that by talking to their leader, the NCR would attack the prison.

Is there any way I can reset the quest (on PC) so that I can assist the NCR?

Other than going back to an old save, no.

That svcks

There is a command: "resetquest" hat you can use, but I think you can only use it if you havent completed the quest. In order to use the command you have to specify the hex value of the quest: http://fallout.wikia.com/wiki/Fallout:New_Vegas_Quest_IDs

You can then use the setstage command to put yourself back at the proper stage of the quest.

resetquest xxxxxxxx
setstage xxxxxxxx ###