mxser: move board init into mxser_initbrd

What's done in ->probe for board is actually its initialization. So move
it to mxser_initbrd where it belongs.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-44-jslaby@suse.cz


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