staging: iio: ad9951: Use devm_iio_device_register

This patch introduces the use of devm_iio_device_register and does away
with the unregister in the remove function. The remove function is no
longer required and is completely removed.

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>