usbip: simplify the return expression of usbip_core_init()

Simplify the return expression.

Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Liu Shixin <liushixin2@huawei.com>
Link: https://lore.kernel.org/r/20200915032631.1772673-1-liushixin2@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>