ASoC: samsung: i2s: Fix secondary platform device unregistration

This fixes unregistration of the secondary platform device so all
resources are properly released.  Additionally the removal sequence
is corrected so it is in reverse order comparing to probe sequence.
The test against NULL priv->pdev_sec is removed as it is not necessary.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>