staging:iio:magnetometer:ak8975: set power-down mode after reading fuse ROM data

Fuse ROM data access mode is the only mode which does not
transition to power-down mode automatically.

As per the AK8975 data sheet, it is recomended to set the
power-down mode first before attempting to change into
another mode.

Signed-off-by: Leed Aguilar <leed.aguilar@ti.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>