mxser: drop constant board::uart_type

board::uart_type is always set to PORT_16550A. So, use this constant in
the code instead. And drop the mxser_board member.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-10-jslaby@suse.cz


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