usb: fix improper return value when kzalloc fails

The comment says function wa_nep_queue() should return 0 if ok, and <0
errno code on error. However, its implementation always returns 0, even
if the call to kzalloc() fails. As a result, the return value may be
inconsistent with the execution status, which may mislead the callers.
This patch fixes the bug, returning -ENOMEM when the call to kzalloc()
fails.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>