mxser: Documentation, make the docs up-to-date

Remove all the obsolete information from the documentation of mxser
driver. Either it was about the out-of-tree driver, or it was superseded
by udev et al.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-69-jslaby@suse.cz


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