mpu6000: set the default DLFP filter to 42Hz

this allows for apps to ask for slightly higher filters with the
software filter and not have it completely ruined by the on-chip DLPF

src/drivers/mpu6000/mpu6000.cpp

@@ -477,7 +477,7 @@ void MPU6000::reset()
 	// FS & DLPF   FS=2000 deg/s, DLPF = 20Hz (low pass filter)
 	// was 90 Hz, but this ruins quality and does not improve the
 	// system response
-	_set_dlpf_filter(20);
+	_set_dlpf_filter(42);
 	usleep(1000);
 	// Gyro scale 2000 deg/s ()
 	write_reg(MPUREG_GYRO_CONFIG, BITS_FS_2000DPS);