ASoC: core: Fix obscure leak of runtime array

We're currently not freeing card->rtd in cases where the card is
unregistered before being instantiated - convert it to devm_kzalloc() to
make sure that happens.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>