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

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

Fixes: 58757984ca3c ("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>

drivers/media/radio/si470x/Makefile

@@ -2,6 +2,6 @@
 # Makefile for radios with Silicon Labs Si470x FM Radio Receivers
 #
 
-obj-$(CONFIG_RADIO_SI470X) := radio-si470x-common.o
+obj-$(CONFIG_RADIO_SI470X) += radio-si470x-common.o
 obj-$(CONFIG_USB_SI470X) += radio-si470x-usb.o
 obj-$(CONFIG_I2C_SI470X) += radio-si470x-i2c.o