mxser: increase buf_overrun if tty_insert_flip_char() fails

mxser doesn't increase port->icount.buf_overrun at all. Do so if overrun
happens, so that it can be read from the stats.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20211118073125.12283-17-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>