[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/firewire/Makefile

@@ -1,6 +1,6 @@
 obj-$(CONFIG_DVB_FIREDTV) += firedtv.o
 
-firedtv-y := firedtv-avc.o firedtv-ci.o firedtv-dvb.o firedtv-fe.o firedtv-fw.o
+firedtv-y += firedtv-avc.o firedtv-ci.o firedtv-dvb.o firedtv-fe.o firedtv-fw.o
 firedtv-$(CONFIG_DVB_FIREDTV_INPUT)    += firedtv-rc.o
 
 ccflags-y += -Idrivers/media/dvb-core