w1: ds2490 reduce magic numbers

Use a #define for the usb vendor request type, clear the status
byte and use that instead of a magic offset in checking if idle.

Signed-off-by: David Fries <David@Fries.net>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>