driver core: Add comments about returning array counts

The "read array" variants of the device property functions
can be used to return the number of values in an array.
Update the comments to reflect that.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>