Changing default SPI freq to 4MHz

A 4MHz default is more safe. More platforms have a good divisor at 4MHz rather than 6MHz or 7MHz so we are more likely not to violate the 7MHz max SPI speed.

Author: Nathan Seidle

src/ICM_20948.h

@@ -138,7 +138,7 @@ class ICM_20948_I2C : public ICM_20948
 };
 
 // SPI
-#define ICM_20948_SPI_DEFAULT_FREQ 6000000
+#define ICM_20948_SPI_DEFAULT_FREQ 4000000
 #define ICM_20948_SPI_DEFAULT_ORDER MSBFIRST
 #define ICM_20948_SPI_DEFAULT_MODE SPI_MODE0