intel_th: Fix activating a subdevice without a driver

If output subdevice driver is not loaded, activating it will try to
call its ->activate method and crash. Fix this by explicitly checking
for the driver.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Reviewed-by: Laurent Fert <laurent.fert@intel.com>
2 files changed