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>

target/linux/ramips/dts/mt7621_glinet_gl-mt1300.dts

@@ -67,8 +67,7 @@
 	flash@0 {
 		compatible = "jedec,spi-nor";
 		reg = <0>;
-		spi-max-frequency = <80000000>;
-		m25p,fast-read;
+		spi-max-frequency = <50000000>;
 		broken-flash-reset;
 
 		partitions {