Include the driver name in the tty_register_device_attr() error
message for invalid index.

Note that tty_err() cannot be used here because there is no tty;
use pr_err().
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>