n_tty: invert TTY_NORMAL condition in n_tty_receive_buf_standard

n_tty: invert TTY_NORMAL condition in n_tty_receive_buf_standard

Handle !TTY_NORMAL as a short path and 'continue' the loop. Do the rest as
a normal code flow. This decreases the indentation level by one and
makes the code flow more understandable.

IOW, we avoid
  if (cond) {
    LONG CODE;
  } else
    single_line();
by
  if (!cond) {
    single_line();
    continue;
  }
  LONG CODE;

While at it, invert also the 'if (!test_bit) A else B' into 'if
(test_bit) B else A'.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-10-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>