wusb: fix error return code in wusb_prf()

Fix to return error code -ENOMEM from the kmalloc() error handling
case instead of 0, as done elsewhere in this function.

Fixes: a19b882c ("wusb: Stop using the stack for sg crypto scratch space")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>