linux: Properly deal with invalid config.wTotalLength in sysfs

linux: Properly deal with invalid config.wTotalLength in sysfs

In usbfs wTotalLength can be trusted, in the sense that the kernel simple
has holes in the descriptors file when a device returns a smaller config
descriptor then advertised.

In sysfs this is not the case, sysfs descriptors only contain descriptors
actually returned by the device, with no holes. The kernel does validate
the bLength field of all the descriptors and removes any invalid ones.

So with sysfs we cannot rely on wTotalLength, since we can trust bLength,
this patch searches forward for a descriptor with type of LIBUSB_DT_CONFIG
to find the next config on sysfs.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>