ramips: mt7621: reduce MMC clock to 48 MHz

ramips: mt7621: reduce MMC clock to 48 MHz

The switch to the upstream mmc-mtk driver caused problems with MT7621
because of unstable too high clock frequency:
[   49.643291] mmc0: error -88 whilst initialising SD card
[   49.890047] mmc0: error -88 whilst initialising SD card
[   50.142414] mmc0: error -88 whilst initialising SD card
[   50.419218] mmc0: error -88 whilst initialising SD card
...

Fix this by reducing the clock speed to 48 MHz instead of 50 MHz, which
is also the value used in upstream Linux mt7621.dtsi.

With that change applied SD cards work as expected on MT7621 devices
also with the new driver.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>