synclink fix ldisc buffer argument

Fix call to line discipline receive_buf by synclink drivers.
Dummy flag buffer argument is ignored by N_HDLC line discipline but might
be of insufficient size if accessed by a different line discipline
selected by mistake. flag buffer allocation now matches max size of data
buffer. Unused char_buf buffers are removed.

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>