Bluetooth: Convert L2CAP ident spinlock into a mutex

The spinlock protecting the L2CAP ident number can be converted into
a mutex since the whole processing is run in a workqueue. So instead
of using a spinlock, just use a mutex here.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>