iio: hid-sensor-magn-3d: Fix build warning

Fix build warning, sizeof() called on dynamically
sized pointer, by removing the call and the dependent
function parameter. It is not needed or used in this
driver, when pushing values to an iio buffer.

Changes from v1
- Fix mistake in varible name

Signed-off-by: Reyad Attiyat <reyad.attiyat@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>