hwmon: Refactor SENSOR_DEVICE_ATTR_2

This patch refactors SENSOR_DEVICE_ATTR_2 macro, following pattern set by
SENSOR_ATTR.  First it creates a new macro SENSOR_ATTR_2() which expands
to an initialization expression, then it uses that in SENSOR_DEVICE_ATTR_2,
which declares and initializes a struct sensor_device_attribute_2.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>