LM77: correction of "register to temperature"

Temperature conversion in lm77.c is currently broken for negative temperatures, as discovered in
http://list.voyage.hk/pipermail/voyage-linux/2005-December/000540.html

National uses the upper 4 bits of the temperature registers to represent the sign (all these are either set or unset at the same time), while the lower 3 bits are either unused or represent the alarm status (depending on which of the temperature registers you look at).

The "register to temperature" conversion doesn't properly consider these sign bits. In the result they are used as part of the absolute temperature value, so that negative temperatures are incorrectly converted to huge positive values instead.
(Side note: the "temperature to register" conversion works fine with positive and negative values.)

The attached patch (which is against kernel 2.6.14.4) solves this issue. It has been tested against all the sample values given in the LM77 datasheet (section 1.4) to proof it is working correctly.

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