hwmon: (lm78) Detect alias chips

The LM78 and LM79 can be accessed either on the I2C bus or the ISA
bus. We must not access the same chip through both interfaces. So far
we were relying on the user passing the correct ignore parameter to
skip the registration of the I2C interface as suggested by
sensors-detect, but this is fragile: the user may load the lm78
driver without running sensors-detect, and the i2c bus numbers are
not stable across reboots and hardware changes.

So, better detect alias chips in the driver directly, and skip any
I2C chip which is obviously an alias of the ISA chip. This is done
by comparing the value of 26 selected registers.

Signed-off-by: Jean Delvare <khali@linux-fr.org>