kernel: disable CONFIG_BCM_IPROC_ADC by default

This fixes compiling issue on bcm53xx target with kmod-iio-core:
Broadcom IPROC ADC driver (BCM_IPROC_ADC) [N/m/?] (NEW)

Signed-off-by: Rafał Miłecki <[email protected]>

target/linux/generic/config-4.9

@@ -425,6 +425,7 @@ CONFIG_BASE_SMALL=0
 # CONFIG_BCMA_DRIVER_GPIO is not set
 CONFIG_BCMA_POSSIBLE=y
 # CONFIG_BCMGENET is not set
+# CONFIG_BCM_IPROC_ADC is not set
 # CONFIG_BCM_KONA_USB2_PHY is not set
 # CONFIG_BDI_SWITCH is not set
 # CONFIG_BE2ISCSI is not set