staging: iio_simple_dummy: fix init function

This patch fixes the init function for the iio_simple_dummy driver.
The main issues were absence of kfree for the allocated array, and no
devices being removed in case the probe function fails, running in a loop.

Signed-off-by: Vladimirs Ambrosovs <rodriguez.twister@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>