w1: continue slave search where previous left off

Search will detect at most max_slave_count devices per run, if there
are more pick up the next search where the previous left off.

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