[media] smiapp: Remove set_xclk() callback from hwconfig

The clock framework is generally so well supported that there's no reason
to keep this one around.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>