Bluetooth: hci_bcm: Add compatible string for BCM43540

The BCM43540 chip is a 802.11 a/b/g/n/ac + Bluetooth 4.1 combo module.
This patch adds a compatible string match to the serdev driver for the
Bluetooth part of the chip.

Signed-off-by: Abhishek Pandit-Subedi <abhishekpandit@chromium.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>