You can download this driver and extract it to a USB drive.  Plug into the machine together with your installation USB.  When you get to "select driver to install", load the driver from the USB.
I have a T7920 that had a hard drive failure, data backed up so that's not an issue. I therefore have downloaded the Windows Restore iso image to make an installation USB memory stick (I created it using Fedora and it seems to be working). The 7920 boots and after a while starts the Windows re-installation procedure, it get's through the assorted country/language questions, then starts the setup program. After a while it throws up a missing driver 'select driver to install' page, telling me that either the CD, DVD or USB could be missing a driver, however it doesn't tell me what driver is missing. So at the moment I'd somewhat stumped.  
Dell T7920 Windows Restore - Asking for driver during install?