w1: fix w1_send_slave dropping a slave id

Previous logic,
if (avail > 8) {
	store slave;
	return;
}
send data; clear;

The logic error is, if there isn't space send the buffer and clear,
but the slave wasn't added to the now empty buffer loosing that slave
id.  It also should have been "if (avail >= 8)" because when it is 8,
there is space.

Instead, if there isn't space send and clear the buffer, then there is
always space for the slave id.

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