rtw8821cw fails to go into monitor mode

[obbardc]

hi! thanks for your work on this, it's really nice to see support getting into the kernel for these weird and wonderful wifi cards.
I am using the firmware from the linux-firmware tree since its missing from this repo: https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git/tree/rtw88

I've built latest master fine (same as the dev branch) and trying to get into monitor mode for aircrack.

It loads just fine and ends up being able to scan and connect to APs:

# lsusb
Bus 002 Device 002: ID 0bda:c820 Realtek Semiconductor Corp. 

# modprobe mac80211
[   49.397533] cfg80211: Loading compiled-in X.509 certificates for regulatory database
[   49.408691] cfg80211: Loaded X.509 cert 'sforshee: 00b28ddf47aef9cea7'
[   49.409459] platform regulatory.0: Direct firmware load for regulatory.db failed with error -2
[   49.410246] platform regulatory.0: Falling back to sysfs fallback for: regulatory.db
[   49.414004] cfg80211: failed to load regulatory.db

# insmod rtw88_core.ko
[   62.355646] rtw88_core: loading out-of-tree module taints kernel.

# insmod rtw88_usb.ko

# insmod rtw88_8821c.ko

# insmod rtw88_8821cu.ko
[   62.915177] rtw_8821cu 2-1:1.2: USB: 2
[   62.918105] rtw_8821cu 2-1:1.2: Firmware version 24.5.0, H2C version 12
[   63.313457] usbcore: registered new interface driver rtw_8821cu
[   63.316623] rtw_8821cu 2-1:1.2 wlx4401bb8e21c4: renamed from wlan0
[   63.780152] rtw_8821cu 2-1:1.2: failed to download firmware
[   65.187405] rtw_8821cu 2-1:1.2: start vif 44:01:bb:8e:21:c4 on port 0

but attempting to go into monitor mode to scan with aircrack ends up with:

# ip link set wlx4401bb8e21c4 down
[  117.257098] rtw_8821cu 2-1:1.2: stop vif 44:01:bb:8e:21:c4 on port 0

# iw dev wlx4401bb8e21c4 set type monitor

# rfkill unblock all

# ip link set wlx4401bb8e21c4 up
[  151.469571] rtw_8821cu 2-1:1.2: error beacon valid
[  151.470515] rtw_8821cu 2-1:1.2: failed to download rsvd page
[  151.472065] rtw_8821cu 2-1:1.2: failed to download firmware
RTNETLINK answers: Device or resource busy

# airodump-ng wlx4401bb8e21c4
[  661.035071] rtw_8821cu 2-1:1.2: error beacon valid
[  661.036428] rtw_8821cu 2-1:1.2: failed to download rsvd page
[  661.038253] rtw_8821cu 2-1:1.2: failed to download firmware
[  661.045981] device wlx4401bb8e21c4 entered promiscuous mode
ioctl(SIOCSIFFLAGS) failed: Device or resource busy

[ulli-kroll]

#14