w1: ds2408: make value read-back check a Kconfig option

De-activating this reading back will effectively half the time required
for a write to the output register.

Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>