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 -obj rules.

Reported-by: Hans Verkuil
Identified-by: Antti Polosaari
Tested-by: Antti Palosaari
Signed-off-by: Mauro Carvalho Chehab