tmp105: Correct handling of temperature limit checks

The TMP105 datasheet says that in Interrupt Mode (when TM==1) the device
signals an alert when the temperature equals or exceeds the T_high value and
then remains high until a device register is read or the device responds to
the SMBUS Alert Response address, or the device is put into Shutdown Mode.
Thereafter the Alert pin will only be re-signalled when temperature falls
below T_low; alert can then be cleared in the same set of ways, and the
device returns to its initial "alert when temperature goes above T_high"
mode. (If this textual description is confusing, see figure 3 in the
TI datasheet at https://www.ti.com/lit/gpn/tmp105

 .)

We were misimplementing this as a simple "always alert if temperature is
above T_high or below T_low" condition, which gives a spurious alert on
startup if using the "T_high = 80 degrees C, T_low = 75 degrees C" reset
limit values.

Implement the correct (hysteresis) behaviour by tracking whether we
are currently looking for the temperature to rise over T_high or
for it to fall below T_low. Our implementation of the comparator
mode (TM==0) wasn't wrong, but rephrase it to match the way that
interrupt mode is now handled for clarity.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20201110150023.25533-3-peter.maydell@linaro.org