Revert "HAL_ChibiOS: lower speed of mpu9250"

This reverts commit a4e51f3942.

This commit led to more errors on the first IMU on a normal
CubeBlack. I don't yet know why this happens, but I'm removing this in
master for now as it affects primary IMU

libraries/AP_HAL_ChibiOS/hwdef/fmuv3/hwdef.dat

@@ -403,7 +403,7 @@ SPIDEV ms5611         SPI1 DEVID3  BARO_CS      MODE3 20*MHZ 20*MHZ
 SPIDEV ms5611_ext     SPI4 DEVID2  BARO_EXT_CS  MODE3 20*MHZ 20*MHZ
 SPIDEV mpu6000        SPI1 DEVID4  MPU_CS       MODE3  2*MHZ  8*MHZ
 SPIDEV icm20608-am    SPI1 DEVID2  ACCEL_EXT_CS MODE3  4*MHZ  8*MHZ
-SPIDEV mpu9250        SPI1 DEVID4  MPU_CS       MODE3  2*MHZ  4*MHZ
+SPIDEV mpu9250        SPI1 DEVID4  MPU_CS       MODE3  4*MHZ  8*MHZ
 SPIDEV mpu9250_ext    SPI4 DEVID1  MPU_EXT_CS   MODE3  4*MHZ  8*MHZ
 SPIDEV icm20948       SPI1 DEVID4  MPU_CS       MODE3  4*MHZ  8*MHZ
 SPIDEV icm20948_ext   SPI4 DEVID1  MPU_EXT_CS   MODE3  4*MHZ  8*MHZ