mpu9250: set accel DLPF to 41 Hz (PX4#5177)

Previously, the accel DLPF was not set, so it's not clear what the
settings are or should be.

src/drivers/mpu9250/mpu9250.cpp

@@ -169,6 +169,8 @@
 #define BITS_DLPF_CFG_3600HZ		0x07
 #define BITS_DLPF_CFG_MASK		0x07
 
+#define BITS_ACCEL_CONFIG2_41HZ		0x03
+
 #define BIT_RAW_RDY_EN			0x01
 #define BIT_INT_ANYRD_2CLEAR		0x10
 
@@ -471,6 +473,9 @@ int MPU9250::reset()
 	write_checked_reg(MPUREG_INT_PIN_CFG, BIT_INT_ANYRD_2CLEAR); // INT: Clear on any read
 	usleep(1000);
 
+	write_checked_reg(MPUREG_ACCEL_CONFIG2, BITS_ACCEL_CONFIG2_41HZ);
+	usleep(1000);
+
 	uint8_t retries = 10;
 
 	while (retries--) {