n_tty: honor opost flag for echoes

Fixes the following bug:

      http://bugs.linuxbase.org/show_bug.cgi?id=2692

Causes processing of echoed characters (output from the echo buffer) to
honor the O_OPOST flag, which is consistent with the old behavior.

Note that this and the next patch ("n_tty: move echoctl check and
clean up logic") were verified together by the bug reporters, and
the test now passes.

Signed-off-by: Joe Peterson <joe@skyrush.com>
Cc: Linux Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>