tests/periph/spi_dma: fix Makefile.ci

The `Makefile.ci` was intended to be read and generated by command
line utilities and the contents should look exactly like this:

```
BOARD_INSUFFICIENT_MEMORY += \
    board_a \
    board_b \
    ... \
    #
```

No fancy schmancy, no `include`, no nothing. This fixes the format.