mxser: move max_baud from port to board

mxser_port::max_baud is actually a board property, not port's. So move
it to mxser_board.

While at it, drop the printk informing about max baud rate during probe.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-11-jslaby@suse.cz


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