tty: mxser: fix cycle termination condition in mxser_probe() and mxser_module_init()

There is a bug in resources deallocation code in mxser_probe() and
mxser_module_init().  As soon as variable 'i' is unsigned int, cycle
termination condition i >= 0 is always true.  The patch fixes the issue.

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>