[media] smiapp: Don't compile of_read_number() if CONFIG_OF isn't defined

of_read_number() is defined in of.h but does not return an error code, so
that non-of implementation could simply return an error.

Temporarily work around this until of_read_number() can be replaced by
of_property_read_u64_array().

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>