USB: add rationale on why usb descriptor structures have to be packed

Add argumentation in defense of using __attribute__((packed)) in USB
descriptors authored by Dave Brownell. Necessary as in some cases it
seems superfluous.

Signed-off-by: Inaky Perez-Gonzalez <inaky@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>