w1: use family_data instead of rom in w1_slave

The first line printed from w1_slave gives the context of the w1
device.  So does the second line, but if the CRC check failed, the
second line contains the last successful result.  It is confusing when
it prints the temperature next to the line that might be a previous
conversion and has nothing to do with that printed temperature value.
Modify the code to store the last good conversion in family_data,
which is designed for custom data structures.

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