media: sti: fix obj-$(config) targets

The right thing to do is to add a new object to the building
system when a certain config option is selected, and *not*
override them.

So, fix obj-$(config) logic at sti makefiles, using "+=",
instead of ":=".

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>