intel_th: msu: Fix an uninitialized mutex

Commit 615c164d ("intel_th: msu: Introduce buffer interface") added a
mutex that it forgot to initialize, resulting in a lockdep splat.

Fix that by initializing the mutex statically.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Fixes: 615c164d ("intel_th: msu: Introduce buffer interface")
Link: https://lore.kernel.org/r/20191028070651.9770-3-alexander.shishkin@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>