linux: Clear descriptor buffer before read when using usbfs

linux: Clear descriptor buffer before read when using usbfs

In usbfs the config descriptors are config.wTotalLength bytes apart, but the
device may actually return a shorter descriptor then advertised, in this
case the kernel will simply skip over any bytes read which the device did
not actually return. Note the kernel will *not* return 0 data, it will simply
leave the memory passed in to the read call as is.

Therefor this patch clears the buffer before calling read, to ensure that
the non existing parts of the descriptors are always properly zero-ed.

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