Bug#1028547: MT7921e firmware blob exists but is not found

[Jamie Wilkinson <jaq@debian.org>]

I am installing on an Asus PN51-E1 which includes the Mediatek MT7921e
wifi device.

I came across this bug while trying to install from
https://cdimage.debian.org/cdimage/bookworm_di_alpha2/ netinst iso on
a USB stick.

I see the same error, but what I was trying to debug when I arrived at
this bug is why we see the error in dmesg:

mt7921e 0000:05:00.0: firmware: failed to load
mediatek/WIFI_MT7961_patch_mcu_1_2_hdr.bin (-2)

because when I break to the console in the installer, I see in
/lib/firmware this very file:

~ # ls /lib/firmware/mediatek/WIFI_*
WIFI_MT7922_patch_mcu_1_1_hdr.bin
WIFI_MT7961_patch_mcu_1_2_hdr.bin
WIFI_RAM_CODE_MT7922_1.bin
WIFI_RAM_CODE_MT7961_1.bin

When I tried again in Expert install mode, I looked before detecting
network devices, and saw that /lib/firmware was empty.  What's
populating this directory?  Is it a chroot and thus the kernel module
can't see it?  Or... is the kernel module looking in a different path?