mxser: extract and dedup CTS handling

CTS is handled on two locations in mxser. Extract the code into
separation function mxser_handle_cts. And call it from the two
locations.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-59-jslaby@suse.cz


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