mxser: integrate mxser.h into .c

As mxser.h serves only mxser.c, integrate the definitions into .c and
drop .h. There are some "semi-public" ioctl definitions, but they are
mostly go away in the next patches. If something, they would need to
live in an uapi header anyway.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-5-jslaby@suse.cz


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