USB: usb.h: tweak struct urb to remove wasted space

By moving one field around in 'struct urb' we reduce the size of the
structure by 8 bytes.

Before the patch on x86_64 the overall size of the structure as reported
by pahole was:
	/* size: 192, cachelines: 3, members: 30 */
	/* sum members: 184, holes: 2, sum holes: 8 */
After the patch we now have:
	/* size: 184, cachelines: 3, members: 30 */
	/* last cacheline: 56 bytes */

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>