staging: usbip: usbip_common.h: reorganize and document request headers

Document the request header structures; move #defines out of the
structures; organize function declarations by source file; and move
inline functions to the end of file.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>