ASoC: Intel: do cast earlier in sst_cdev_tstamp()

My static checker complains about these because it looks like the
multiply can overflow and then we cast to a larger data type.  I don't
think this is a problem, but it's also harmless to do the cast earlier
so let's silence the static checker warning.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>