The drvdata for iio-parent devices points to the struct iio_dev for
the iio-device. So by directly casting the return from i2c_get_clientdata()
to struct bmc150_accel_data * the code was ending up storing the second_dev
pointer in (and retrieving it from) some semi-random offset inside
struct iio_dev, rather then storing it in the second_dev member of the
bmc150_accel_data struct.

Fix the code to get the struct bmc150_accel_data * pointer to call
iio_priv() on the struct iio_dev * returned by i2c_get_clientdata(),
so that the correct pointer gets dereferenced.

This fixes the following oops on rmmod, caused by trying to
dereference the wrong return of bmc150_get_second_device():

[  238.980737] BUG: unable to handle page fault for address: 0000000000004710
[  238.980755] #PF: supervisor read access in kernel mode
[  238.980760] #PF: error_code(0x0000) - not-present page
...
[  238.980841]  i2c_unregister_device.part.0+0x19/0x60
[  238.980856]  0xffffffffc0815016
[  238.980863]  i2c_device_remove+0x25/0xb0
[  238.980869]  __device_release_driver+0x180/0x240
[  238.980876]  driver_detach+0xd4/0x120
[  238.980882]  bus_remove_driver+0x5b/0xd0
[  238.980888]  i2c_del_driver+0x44/0x70

While at it also remove the now no longer sensible checks for data
being NULL, iio_priv never returns NULL for an iio_dev with non 0
sized private-data.

Fixes: 5bfb3a4b ("iio: accel: bmc150: Check for a second ACPI device for BOSC0200")
Cc: Jeremy Cline <jeremy@jcline.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>