n_tty: move echoctl check and clean up logic

Check L_ECHOCTL before insertting a character in the echo buffer
(rather than as the buffer is processed), to be more consistent with
when all other L_ flags are checked.  Also cleaned up the related logic.

Note that this and the previous patch ("n_tty: honor opost flag for echoes")
were verified together by the reporters of the bug that patch addresses
(http://bugs.linuxbase.org/show_bug.cgi?id=2692), and the test now passes.

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