staging:iio:magnetometer:ak8975: remove 'mode' attribute

The raw data reading process (ak8975_read_axis) sets the
single measurement mode to take a sample and once the
conversion is completed the device enters into power-down
mode automatically, therefore there is no need to enable
a flag for this to happen.

Signed-off-by: Leed Aguilar <leed.aguilar@ti.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>