hwmon: (tmp102) Force wait for conversion time for the first valid data

TMP102 works based on conversions done periodically. However, as per
the TMP102 data sheet[1] the first conversion is triggered immediately
after we program the configuration register. The temperature data
registers do not reflect proper data until the first conversion is
complete (in our case HZ/4).

The driver currently sets the last_update to be jiffies - HZ, just
after the configuration is complete. When TMP102 driver registers
with the thermal framework, it immediately tries to read the sensor
temperature data. This takes place even before the conversion on the
TMP102 is complete and results in an invalid temperature read.

Depending on the value read, this may cause thermal framework to
assume that a critical temperature event has occurred and attempts to
shutdown the system.

Instead of causing an invalid mid-conversion value to be read
erroneously, we mark the last_update to be in-line with the current
jiffies. This allows the tmp102_update_device function to skip update
until the required conversion time is complete. Further, we ensure to
return -EAGAIN result instead of returning spurious temperature (such
as 0C) values to the caller to prevent any wrong decisions made with
such values. NOTE: this allows the read functions not to be blocking
and allows the callers to make the decision if they would like to
block or try again later. At least the current user(thermal) seems to
handle this by retrying later.

A simpler alternative approach could be to sleep in the probe for the
duration required, but that will result in latency that is undesirable
and delay boot sequence un-necessarily.

[1] http://www.ti.com/lit/ds/symlink/tmp102.pdf



Cc: Eduardo Valentin <edubezval@gmail.com>
Reported-by: Aparna Balasubramanian <aparnab@ti.com>
Reported-by: Elvita Lobo <elvita@ti.com>
Reported-by: Yan Liu <yan-liu@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>