iio: accel: hid-sensor-accel-3d: Fix duplicate scan index error
When both accel_3d and gravity sensor are present, iio_device_register()
fails with "Duplicate scan index" error.
The reason for this is setting of indio_dev->num_channels based on
accel_3d channel for both gravity and accel-3d sensor. But number of
channels are not same, so for gravity it is pointing to some invalid
memory and getting scan_index to compare which may match.
To fix this issue, set the indio_dev->num_channels correctly based on
the sensor type.