Bluetooth: Controller: Use BT_HCI_ERR_UNSPECIFIED as needed

A Host shall consider any error code that it does not
explicitly understand equivalent to the error code
Unspecified Error (0x1F).

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>
(cherry picked from commit 78466c8f)
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>