Bluetooth: hci_bcm: Make shutdown and device wake GPIO optional

According to the devicetree binding the shutdown and device wake
GPIOs are optional. Since commit 3e81a4ca ("Bluetooth: hci_bcm:
Mandate presence of shutdown and device wake GPIO") this driver
won't probe anymore on Raspberry Pi 3 and Zero W (no device wake GPIO
connected). So fix this regression by reverting this commit partially.

Fixes: 3e81a4ca ("Bluetooth: hci_bcm: Mandate presence of shutdown and device wake GPIO")
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>