Bluetooth: Remove return value from hci_send_frame() function

The return value of hci_send_frame() is never checked. So just make
this function void and print an error when the hdev->send driver
callback returns a negative value.

Having the error printed is actually an improvement over the
current situation where any driver error just gets ignored.

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