usbip: tools: Fix read_usb_vudc_device() error path handling

This isn't really accurate right. fread() doesn't always
return 0 in error. It could return < number of elements
and set errno.

Signed-off-by: GwanYeong Kim <gy741.kim@gmail.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20191018032223.4644-1-gy741.kim@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>