mxser: make mxser_board::idx really an index

mxser_board::idx is named as index, but actually contain a base. So
assign index there. And for base, introduce a local variable.

This is needed for the next patch (use index as a bit index).

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210618061516.662-52-jslaby@suse.cz


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