initialize pcm_config to zero

sw_params ioctl() is failing with 'invalid parameter' error
since pointer to pcm_config passed from tinyplay is not zero
initialized, hence invalid values are passed for some
parameters.

authored-by: Shiv Maliyappanahalli <smaliyap@codeaurora.org>

Bug: 23525433
Change-Id: I841f29c82ec3fb7646ad8c47fca25963b5f77945