ath79: also reduce spi-max-frequency for buffalo_wzr-hp-ag300h

ath79: also reduce spi-max-frequency for buffalo_wzr-hp-ag300h

In accordance to ebc090e420d1 ("ath79: reduce spi-max-frequency to 50 MHz")
this also reduces the spi-max-frequency to 50 MHz for the last remaining
device with higher frequency in ath79. This will save us from having a
single special case that will require adjustment when the spi driver for
this device is changed in the future.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Acked-by: Chuanhong Guo <gch981213@gmail.com>