orinoco: avoid assertion in case of NULL pointer

In ezusb_init, if upriv is NULL, the code crashes. However, the caller
in ezusb_probe can handle the error and print the failure message.
The patch replaces the BUG_ON call to error return.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>