SB1000/SB2000 now quieter

[Julian Coleman <jdc%coris.org.uk@localhost>]

Hi,

I've added sysmon support to tda(4), the driver for the fans on the SB1000
and SB2000 [1].  `envstat` now reports the speed-related values, for example:

                Current  CritMax  WarnMax  WarnMin  CritMin  Unit
  [admtemp0]
    internal:    22.000                                      degC
    external:    60.000                                      degC
  [admtemp1]
    internal:    21.000                                      degC
    external:    59.000                                      degC
  [tda0]
     fan.cpu:        27        0        0        0        0  none
     fan.sys:        22        0        0        0        0  none

When adding this, I noticed that the value for `fan.sys' was always 63 (full
speed).  A bug in the speed setting logic was causing this, and that is also
fixed with this commit.  The result is that my SB2000 is now much quieter
than it was previously, and I can see the speeds of the fans vary with the
temperatures.

I've also added a detach routine used at shutdown time, which sets the fans
speeds to maximum [2].  I based this on the firmware setting the fans speeds
to maximum, as the temperatures are not being monitored.

Thanks,

J

  [1] http://mail-index.NetBSD.org/source-changes/2013/02/02/msg041146.html
  [2] http://mail-index.NetBSD.org/source-changes/2013/02/03/msg041167.html

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