Bluetooth: Move HCI request internals to net/bluetooth/hci_core.c

The internals of the HCI request framework should not be leaking to
its users. Move them all into net/bluetooth/hci_core.c and provide
a simple hci_req_pending helper function for the one user outside
the framework.

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