ALSA: Ensure PXA runtime data is initialised

The rest of the code relies on the runtime data being zero initialised
so we need to use kzalloc() to allocate it.

Reported-by: Oliver Ford <ipaqlinux@oliford.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>