adt7470: make automatic fan control really work

It turns out that the adt7470's automatic fan control algorithm only works
when the temperature sensors get updated.  This in turn happens only when
someone tells the chip to read its temperature sensors.  Regrettably, this
means that we have to drive the chip periodically.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Cc: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>