Buy a flasher or find someone in the forums offering a downgrade service near you...
http://www.ps3hax.net/showthread.php?t=39766
What disc was in the drive? ...because it wouldnt be a disc requiring 4.31 OFW (that would accidentally update you to 4.31 OFW) unless you were playing the latest DMC disc....
EDIT: whatever the FW was on the disc is your true FW, my guess is the rogero spoof 4.31 is intact but the FW installed is whatever the disc holds, you might have been lucky and installed a FW under 3.55
