staging: iio: ak8975: Remove i2c client data corruption

i2c client data set is of type struct indio_dev pointer and hence the
pointer returned from i2c_get_clientdata() should be assigned to
an object of type struct indio_dev and not to an object of type
struct ak8975_data.

Also in ak8975_probe() client data should be set first
before calling ak8975_setup() as it references the client data.

Signed-off-by: Preetham Chandru R <pchandru@nvidia.com>
CC: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>