Bluetooth: btbcm: Add missing static inline in header

This fixes a double definition error when CONFIG_BT_BCM is not set.

Fixes: 52837990 ("Bluetooth: btbcm: Support pcm configuration")
Signed-off-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>