Bluetooth: Move synchronous request handling into hci_request.c

hci_request.c is a more natural place for the synchronous request
handling. Furthermore, we will soon need access to some of the
previously private-to-hci_core.c functions from hci_request.c.

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