iio: imu: inv_mpu6050: temperature only work with accel/gyro

Temperature sensor works correctly only when accel and/or gyro
is turned on. Prevent polling value if they are not running.
Anyway it doesn't make sense to use it without sensor engines
on.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>