iio: hid-sensor-magn-3d: Scan for usage attributes before setting up iio channels

Scan for and count the HID usage attributes supported by the driver.
This allows for the driver to only setup the IIO channels for the
sensor usages present in the HID USB reports.

Changes from v5
-Fixed kernel panic from invalid pointer dereference
-Fixed variable assignment style

Signed-off-by: Reyad Attiyat <reyad.attiyat@gmail.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>