[media] smiapp: Use SMIAPP_PADS when referring to number of pads

Replace plain value 2 with SMIAPP_PADS when referring to the number of
pads.

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>