HID: hid-sensor-hub: Add selector api

In some report descriptors, they leave holes in the selectors. In
this case if we use hardcoded selector values, this will result
in invalid values. For example, if there is selectors defined for
Power State from OFF to D0 to D3. We can't use indexes of these states
if some states are not implemented or not present in the report decriptors.
In this case, we need to get the indexes from report descriptors.

One API is added to get the index of a selector. This API will
search for usage id in the field usage list and return the index.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>