firmware: raspberrypi: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], this
removes the VLA in favor of a maximum size and adds a sanity check.
Existing callers of the firmware interface never need more than 24
bytes (struct gpio_set_config). This chooses 32 just to stay ahead
of future growth.

v2: Fix the length passed to rpi_firmware_property_list (by anholt,
    acked by Kees).

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com



Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>