hwmon: Allow sensor attributes arrays

This patch refactors SENSOR_DEVICE_ATTR macro.  First it creates a new
macro SENSOR_ATTR() which expands to an initialization expression, then
it uses that in SENSOR_DEVICE_ATTR, which declares and initializes a
struct sensor_device_attribute.

IOW, SENSOR_ATTR() imitates __ATTR() in include/linux/device.h.

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