staging: vc04_services: make BCM_VIDEOCORE tristate

Adding the 'bool' symbol brought back a randconfig build bug that
I had fixed before:

drivers/staging/built-in.o: In function `vchiq_probe':
(.text+0x1da30): undefined reference to `rpi_firmware_get'
drivers/staging/built-in.o: In function `vchiq_platform_init':
(.text+0x27494): undefined reference to `rpi_firmware_property'

The problem is that when RASPBERRYPI_FIRMWARE is a loadable module,
but BCM2835_VCHIQ can again be built-in. Making BCM_VIDEOCORE
itself tristate will make Kconfig honor the dependency correctly.

Fixes: 6bbfe4a7 ("staging: vc04_services: Create new BCM_VIDEOCORE setting for VideoCore services.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>