input: misc: ixp4-beeper: use gpiolib strictly

Request and free the GPIO line used for the beeper properly.
Then use the gpiolib API to flip the output of the GPIO pin
instead of relying on hacks to poke the register bits.

Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Alexandre Courbot <acourbot@nvidia.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Krzysztof Halasa <khc@pm.waw.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>