As L5s have been shipping with the SparkLAN card for some time now, and therefore with the required firmware installed, I imagine that the current builds posted for flashing contain all you need.
In my comment #8 to you, I linked a thread in which @dos (Purism developer) said that the presence of the firmware (or not) can be checked in the L5’s file system:
If, after you have reflashed the OS on the L5, you find firmware files in /usr/share/firmware-librem5-nonfree/firmware/brcm, then you’re good to go. If not, then you’ll need to download and install it.