n_hdlc: cleanup messages during registration

1) n_hdlc prints two lines during registration. Squeeze it into one.
2) prefix the error message with "N_HDLC: ", so that it's clear which
   ldisc failed to register.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084118.26491-5-jslaby@suse.cz


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