iio: imu: inv-mpu9150: fix irq ack preventing irq storms

iio: imu: inv-mpu9150: fix irq ack preventing irq storms

IRQ needs to be acked. for some odd reasons, reading from irq status does
not reliable help, enable acking from any register to be on the safe side
and read the irq status register. Comments in the code indicate a known
unreliability with that register.
The blamed commit was tested with mpu6050 in lg,p895 and lg,p880 according
to Tested-bys. But with the MPU9150 in the Epson Moverio BT-200 this leads
to irq storms without properly acking the irq.

Fixes: 0a3b517c8089 ("iio: imu: inv_mpu6050: fix interrupt status read for old buggy chips")
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Acked-by: Jean-Baptiste Maneyrol <jean-baptiste.maneyrol@tdk.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>