The introduction of ebf0d8dade (ath79: add new ar934x spi driver)
made the SPI memory unusable. Reducing the spi-max-frequency to
a smaller value makes it work again.
Tested on two MikroTik RouterBOARD wAP G-5HacT2HnD devices.
Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>