ws eink: set max SPI speed to 4MHz

experimental/waveshare_eink/linux_kernel_modules/TODO.md

 # Waveshare E-Ink Driver TODO
 
 ## Questions
-What is the purpose of the `#ifdef __LITTLE_ENDIAN` sections in the driver? Add code comments to
-clarify.
 
 
 ## Fixes Requested
-* I think `eink_device->max_speed_hz` is still being initialized incorrectly. It should be
-  initialized with the max speed supported by the device, not the max speed supported by the SPI
-  master.
 
 
 ## Enhancements Requested

experimental/waveshare_eink/linux_kernel_modules/mangOH_red_ws213.c

@@ -4,7 +4,11 @@
 
 #define SPI_BUS		32766
 #define SPI_BUS_CS1	1
-#define SPI_BUS_SPEED	12000000
+/*
+ * The minimum clock cycle duration supported by the waveshare 2.13 Inch display
+ * is 250ns which translates to 4MHz bus speed
+ */
+#define SPI_BUS_SPEED	4000000
 
 const char eink_device_name[] = "waveshare_213";
 static struct spi_device *eink_device;