ramips: gl-mt1300: downclock SPI to 50MHz

ramips: gl-mt1300: downclock SPI to 50MHz

The SPI max frequency was set to 80MHz, considerably higher than the
vendor clocks it in their firmware (10MHz).  Multiple users reported
jffs2 corruption/instability in GitHub issue #10461.

My unit has a W25Q256; datasheet specifies maximum SPI frequency for
read command of 50MHz.

Thanks to @DragonBlueP for suggesting to eliminate m25p,fast-read;
and @MPannen1979 for identifying the problem.

Fixes: #10461
Signed-off-by: Michael Lyle <mlyle@lyle.org>
(cherry picked from commit 961e01fc67e7d9e60557df3474fa326216aa4839)