hwmon: (ltc2990) support all measurement modes

Updated version of the ltc2990 driver which supports all measurement
modes (current, voltage, temperature) available in the chip.

If devicetree is used, the mode must be specified with the property
"lltc,meas-mode". The format and possible values of the property are
described in the binding.

If devicetree is not used, the mode of the chip will not be configured.
Unless the chip is configured by another source, only the internal
temperature and supply voltage will be measured.

Signed-off-by: Tom Levens <tom.levens@cern.ch>
Tested-By:  <mike.looijmans@topic.nl>
[groeck: Fixed compiler warning]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>