hwmon: (lm73} Detect and report i2c bus errors

If an LM73 device does not exist on an I2C bus, attempts to communicate
with the device result in an error code returned from the i2c read/write
functions.  The current lm73 driver casts that return value from a s32
type to a s16 type, then converts it to a temperature in celsius.
Because negative temperatures are valid, it is difficult to distinguish
between an error code printed to the response buffer and a negative
temperature recorded by the sensor.

The solution is to evaluate the return value from the i2c functions
before performing any temperature calculations.  If the i2c function did
not succeed, the error code should be passed back through the virtual
file system layer instead of being printed into the response buffer.

Before:

   $ cat /sys/class/hwmon/hwmon0/device/temp1_input
   -46

After:

   $ cat /sys/class/hwmon/hwmon0/device/temp1_input
   cat: read error: No such device or address

Signed-off-by: Chris Verges <kg4ysn@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>