mxser: don't flush buffer from mxser_close() directly

mxser: don't flush buffer from mxser_close() directly

I fail to see the point of calling mxser_flush_buffer() from
mxser_close():
1) The SW xmit buffer is freed in mxser_shutdown_port() right after the
   call to mxser_flush_buffer().  And all 'cnt', 'head', and 'tail' are
   properly initialized to 0 in mxser_activate().
2) The HW buffer is flushed in mxser_shutdown_port() via
   mxser_disable_and_clear_FIFO() too.

So the effect of doing it by mxser_flush_buffer() in mxser_close() is
none. Hence remove it, so that when we use tty_port_close() later, the
code is 1:1 identical.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118073125.12283-10-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>