mxser: alloc only needed # of ports

Many boards have only 2 ports. But we always allocate 8 ports (struct
mxser_port) in each struct mxser_board.

Switch this to an empty/variable sized array ([]) and allocate the
needed number of ports using struct_size helper in ->probe.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-54-jslaby@suse.cz


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