[MPU9250] Added new defines to hw_mpu9250.h and corrected a mistake on #define MPU9250_CONFIG_DLPF_CFG_3600_4000. Implemented MPU9250_Init function for MPU9250 (ACCEL and GYRO, MAG pending).
[MS5611] Implemented MS5611_Init function.
[SPI] Handle structure received by SPI_Write is now passed as pointer to reduce overhead. SPI_Slave structure has now const elements to avoid unintended modifications. Implemented SPI_ClearFIFO and SPI_ReadFIFO.