usb-serial: only check speed once at realize time

Whatever the chardev is open or not, we should assure
the speed is matched each other. So, call usb_check_attach()
check speed. And then pass &error_abort at all calls to
usb_device_attach().

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
