media: si470x: fix a typo at the Makefile causing build issues

Instead of +=, the rule had :=, with actually disables build
of everything else.

Fixes: 58757984 ("media: si470x: allow build both USB and I2C at the same time")
Reported-by: Daniel Scheller <d.scheller.oss@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>