iio: imu: inv_mpu6050: replace timestamp fifo by generic timestamp

Using a fifo for storing timestamps is useless since the interrupt
is in one-shot mode, preventing the hard irq handler to be called
when the irq thread is running. Instead use the generic timestamp
function iio_pollfunc_store_time.

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