iio: adc: ti-ads1015: use iio_device_claim_direct_mode()

While the iio buffer for the ti-ads1015 driver is enabled, reading the
raw ADC channel data is restricted.  We usually use the
iio_device_claim_direct_mode()/iio_device_release_direct_mode() pair for
that.

This change consequently reverses the locking order for the driver's
private lock and indio_dev->mlock which acquired by
iio_device_claim_direct_mode() internally. But it's safe because there is
no other dependency between these locks.

Cc: Daniel Baluta <daniel.baluta@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>