USB: uas: Fix up the Sense IU

Add a comment to the Sense IU data structure that it's also used for Read
Ready and Write Ready.  Remove the 'service response' element since it's
gone from the current draft (04).

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>