tty: n_hdlc add buffer flushing

Add flush_buffer tty callback to flush rx buffers.
Add TCFLSH ioctl processing to flush tx buffers.
Increase default tx buffers from 1 to 3.
Remove unneeded flush_buffer call in open callback.
Remove vendor specific CVS version string.

Signed-off-by: Paul Fulghum <paulkf@microgate.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>