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The hdev->send driver callback is mandatory to be provided by a driver
before calling hci_register_dev. So enforce it and return EINVAL in
case it is not available.

All existing drivers are providing this callback anyway, so this is
just an extra sanity check.
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>