hwmon: (lm75) Speed up detection

Make the LM75/LM75A device detection faster:

* Don't read the current temperature value when we don't use it.
* Check for unused bits in the configuration register as soon as we
  have read its value.
* Don't use word reads, not all devices support this, and some which
  don't misbehave when you try.
* Check for cycling register values every 40 register addresses
  instead of every 8, it's 5 times faster and just as efficient.

Some of these improvements come straight from the user-space
sensors-detect script, so both detection routines are in line now.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Len Sorensen <lsorense@csclub.uwaterloo.ca>
Acked-by: Guenter Roeck <guenter.roeck@ericsson.com>