HAL_Chibios: adjust MPU9250 clock speed down

this helps with fast sampling enabled

libraries/AP_HAL_ChibiOS/hwdef/fmuv3/hwdef.dat

@@ -131,7 +131,7 @@ PE15 VDD_5V_PERIPH_OC INPUT
 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  1*MHZ  8*MHZ 
-SPIDEV mpu9250        SPI1 DEVID4  MPU_CS       MODE3  1*MHZ  8*MHZ 
+SPIDEV mpu9250        SPI1 DEVID4  MPU_CS       MODE3  4*MHZ  4*MHZ 
 SPIDEV mpu9250_ext    SPI4 DEVID1  MPU_EXT_CS   MODE3  4*MHZ  4*MHZ
 SPIDEV hmc5843        SPI1 DEVID5  MAG_CS       MODE3 11*MHZ 11*MHZ
 SPIDEV lsm9ds0_g      SPI1 DEVID4  GYRO_EXT_CS  MODE3 11*MHZ 11*MHZ