iio: imu: inv_mpu6050: fix possible deadlock between mutex and iio

Detected by kernel circular locking dependency checker.

We are locking iio mutex (iio_device_claim_direct_mode) after
locking our internal mutex. But when the buffer starts, iio first
locks its mutex and then we lock our internal one.

To avoid possible deadlock, we need to use the same order
everwhere. So we change the ordering by locking first iio mutex,
then our internal mutex.

Fixes: 68cd6e5b

 ("iio: imu: inv_mpu6050: fix lock issues by using our own mutex")
Cc: stable@vger.kernel.org
Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>