usbd_bulk_transfer returns 1 (USBD_IN_PROGRESS) ?!

[Seb]

Hello,
The function usbd_bulk_transfer() returns sometimes USBD_IN_PROGRESS at random 
when I try to send data to my wireless device (Siemens Gigaset USB Adapter 
54 ; the USB data is processed by the NET2280 chipset). AFAIK, this function 
is never supposed to return USBD_IN_PROGRESS... is it ?
Then, when I unplug the device or try to reboot, sometimes the machine 
completely locks up without any error message (Giant mutex never released in 
the USB driver ?)
Can this be because of a bug in my driver (available at 
http://lekernel.host.lya-network.biz), a hardware problem (I tried with and 
without EHCI on an Intel 82845GV integrated chipset, same bug) or a bug in 
the USB driver ?
Thanks,
Sebastien