gpio: raspberrypi-ext: fix firmware dependency

When the firmware driver is a loadable module, the gpio driver cannot be
built-in:

drivers/gpio/gpio-raspberrypi-exp.o: In function `rpi_exp_gpio_set':
gpio-raspberrypi-exp.c:(.text+0xb4): undefined reference to `rpi_firmware_property'
drivers/gpio/gpio-raspberrypi-exp.o: In function `rpi_exp_gpio_get':
gpio-raspberrypi-exp.c:(.text+0x1ec): undefined reference to `rpi_firmware_property'
drivers/gpio/gpio-raspberrypi-exp.o: In function `rpi_exp_gpio_get_direction':
gpio-raspberrypi-exp.c:(.text+0x360): undefined reference to `rpi_firmware_property'
drivers/gpio/gpio-raspberrypi-exp.o: In function `rpi_exp_gpio_get_polarity':
gpio-raspberrypi-exp.c:(.text+0x4d4): undefined reference to `rpi_firmware_property'
drivers/gpio/gpio-raspberrypi-exp.o: In function `rpi_exp_gpio_dir_out':
gpio-raspberrypi-exp.c:(.text+0x670): undefined reference to `rpi_firmware_property'
drivers/gpio/gpio-raspberrypi-exp.o:gpio-raspberrypi-exp.c:(.text+0x7fc): more undefined references to `rpi_firmware_property' follow
drivers/gpio/gpio-raspberrypi-exp.o: In function `rpi_exp_gpio_dir_in':
drivers/gpio/gpio-raspberrypi-exp.o: In function `rpi_exp_gpio_probe':
gpio-raspberrypi-exp.c:(.text+0x93c): undefined reference to `rpi_firmware_get'

We already have a Kconfig dependency for it, but when compile-testing, it
is disregarded.

This changes the dependency so that compile-testing is only done when the
firmware driver is completely disabled.

Fixes: a98d90e7 ("gpio: raspberrypi-exp: Driver for RPi3 GPIO expander via mailbox service")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>