gpio: raspberrypi-exp: decrease refcount on firmware dt node

We're getting a reference RPi's firmware node in order to be able to
communicate with it's driver. We should decrease the reference count on
the dt node after being done with it.

Fixes: a98d90e7

 ("gpio: raspberrypi-exp: Driver for RPi3 GPIO expander via mailbox service")
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>