Bluetooth: hci_bcm: Replace spinlock by mutex

Replace spinlock by mutex to be able to use bcm_device_lock in
sleepable context like devm_request_threaded_irq or upcomming PM support.

Signed-off-by: Frederic Danis <frederic.danis@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>