[int13] Allow drive to be hooked using the natural drive number

Interpret the maximum drive number (0xff for hard disks, 0x7f for
floppy disks) as meaning "use natural drive number".

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/sanboot.h

@@ -70,7 +70,7 @@ unsigned int san_default_drive ( void );
  *
  * @v uri		URI
  * @v drive		Drive number
- * @ret rc		Return status code
+ * @ret drive		Drive number, or negative error
  */
 int san_hook ( struct uri *uri, unsigned int drive );