n_hdlc: invert conditions in n_hdlc_tty_close and n_hdlc_tty_poll

This makes the functions return immediatelly on invalid state. And we
can push the indent of the later code one level left.

Pass "-w" to "git show" to see we are changing only the conditions (and
whitespace).

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084118.26491-8-jslaby@suse.cz


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