soc: bcm2835: Make !RASPBERRYPI_FIRMWARE dummies return failure

If CONFIG_RASPBERRYPI_FIRMWARE=n:

    drivers/gpio/gpio-raspberrypi-exp.c: In function ‘rpi_exp_gpio_get_polarity’:
    drivers/gpio/gpio-raspberrypi-exp.c:71: warning: ‘get.polarity’ is used uninitialized in this function
    drivers/gpio/gpio-raspberrypi-exp.c: In function ‘rpi_exp_gpio_get_direction’:
    drivers/gpio/gpio-raspberrypi-exp.c:150: warning: ‘get.direction’ is used uninitialized in this function

The dummy firmware interface functions return 0, which means success,
causing subsequent code to make use of the never initialized output
parameter.

Fix this by making the dummy functions return an error code (-ENOSYS)
instead.

Note that this assumes the firmware always fills in the requested data
in the CONFIG_RASPBERRYPI_FIRMWARE=y case.

Fixes: d45f1a56

 ("staging: vc04_services: fix up rpi firmware functions")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>