USB: Fixed outdated usb_get_device_descriptor() documentation

usb_get_device_descriptor() used to convert several descriptor fields to host
CPU's byte order. Now that it doesn't convert them anymore, update the
documentation to reflect this.

Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>