Staging: usbip: Fix the warning of unchecked sscanf return value.

The return value of sscanf in stub_dev.c is not checked. This patch
adds the checking of the return value.

Signed-off-by: Elena Oat <oat.elena@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>