USB: simplify usb_sg_init()

This patch (as1377) simplifies the code in usb_sg_init(), without
changing its functionality.  It also removes a couple of unused fields
from the usb_sg_request structure.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>