n_hdlc: use __func__ and pr_ print helpers

* we mark the message in n_hdlc_tty_receive as error
* we use __func__ instead of explicit function name
* we switch the remaining prints to pr_* helpers

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084118.26491-19-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>