greybus: audio: Fix memory leak

Topology data pointer was mistakenly set to NULL before freeing it. Fix
this.

Fixes: 64a86d9ba850 ("audio: Add module specific driver")
Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>