[media] Fix some Makefile rules

On a few places, := were using instead of +=, causing drivers to
not compile.

While here, standardize the usage of += on all cases where multiple
lines are needed, and for obj-y/obj-m targets, and := when just one
line is needed, on <module>-obj rules.

Reported-by: Hans Verkuil <hverkuil@xs4all.nl>
Identified-by: Antti Polosaari <crope@iki.fi>
Tested-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>