First you have to find out the location of usb firmware from rel-32.3.1 and use below commands to mount and change the usb fw in initrd. If your change takes effect, the dmesg after boot up shall give you a different version number.
mkdir initrd; #
cd initrd;
sudo gunzip -c ../l4t_initrd.img.orig | sudo cpio -i # the default initrd is under Linux_for_Tegra/bootloader
sudo cp ../xusb_sil_prod_fw-new lib/firmware/tegra21x_xusb_firmware
sudo find . | sudo cpio --quiet -H newc -o | sudo gzip -9 -n > ../l4t_initrd.img