Issue while using NRF dongle with HCI_USB firmware

Good afternoon!

I'm trying to run through some of the demo programs that come with Bumble. However, I'm getting an error and the commands hang indefinitely. Ex:
```
akuker@xxx:~/bumble/bumble_src$ bumble-controller-info usb:1
<<< connecting to HCI...
<<< connected
WARNING:bumble.transport.usb:!!! out transfer not completed: status=4
```
```
akuker@xxx:~/bumble/bumble_src$ bumble-scan usb:2fe3:000b
<<< connecting to HCI...
<<< connected
WARNING:bumble.transport.usb:!!! out transfer not completed: status=4
```

Using pyusb gives a slightly different error:
```
akuker@xxx:~/bumble/bumble_src$ bumble-scan pyusb:2FE3:000B
<<< connecting to HCI...
<<< connected
WARNING:bumble.transport.pyusb:USB write error: [Errno 32] Pipe error
```

### My setup:
I'm running Ubuntu 20.04.05 on a reasonably high-end IBM Thinkpad.

I'm using the Nordic nRF52840 dongle with the Zephyr USB_HID demo loaded. I believe this is working properly, since `lsusb` gives reasonable output:
```
akuker@xxx:~$ lsusb
Bus 004 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
Bus 003 Device 028: ID 2fe3:000b ZEPHYR USB-DEV
......
```

Bumble usb probe shows two Bumble-compatible devices:
```
akuker@xxx:~/bumble/bumble_src$ bumble-usb-probe
ID 8087:0032
Bumble Transport Names: usb:0 or usb:8087:0032
Bus/Device: 003/007
Class: Wireless Controller
Subclass/Protocol: 1/1 [Bluetooth]

ID 2FE3:000B
Bumble Transport Names: usb:1 or usb:2FE3:000B or usb:2FE3:000B/912A41BB46CF0B92
Bus/Device: 003/028
Class: Device
Subclass/Protocol: 0/0
Serial: 912A41BB46CF0B92
Manufacturer: ZEPHYR
Product: USB-DEV
```
I'm assuming the host BT stack (BlueZ) is using usb:0/hci0 and not using the Nordic dongle.
```
akuker@xxx:~$ hciconfig
hci0: Type: Primary Bus: USB
BD Address: xx:xx:xx:xx:xx ACL MTU: 1021:4 SCO MTU: 96:6
DOWN
RX bytes:16331 acl:0 sco:0 events:2614 errors:0
TX bytes:641217 acl:0 sco:0 commands:2612 errors:0
````

Do you have any suggestions of things to try to troubleshoot/get to root cause?