[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>

drivers/media/usb/em28xx/Makefile

@@ -1,4 +1,4 @@
-em28xx-y :=	em28xx-video.o em28xx-i2c.o em28xx-cards.o
+em28xx-y +=	em28xx-video.o em28xx-i2c.o em28xx-cards.o
 em28xx-y +=	em28xx-core.o  em28xx-vbi.o
 
 em28xx-alsa-objs := em28xx-audio.o