I have done a bit of googling around. That modem works fine connected to ehternet but it seems you might have problems with the USB side. You might want to try this:http://tldp.org/HOWTO/Motorola-Surfboard-Modem/usb.html
(but its for the 4100-4200 modems so it may not apply)
I could find no evidence here: http://www.qbik.ch/usb/devices/showdevcat.php?id=14&o=140&w=t&s=v&d=a
of the 5120 being shown to work with Linux via USB (but it definetely will with ethernet).
Keep googling, you might stumble on something I didn't. Your easiest route might be to get a $15 ethernet card and connect via that.