iio: adc: ti-ads7950: allow simultaneous use of buffer and direct mode

This modifies the TI ADS7950 A/DC driver to allow the simultaneous use
of both the triggered buffer and reading channels directly (via in-
kernel API or sysfs).

The use case for this is on LEGO MINDSTORMS EV3. Two of the voltage
channels are used in-kernel by a power supply driver, which reads the
values using iio_read_channel_processed(). These channels are only read
at a slow rate (<= 1Hz).  However, we want to be able to read 12 other
channels at the same time using the triggered buffer at a high rate (>=
100Hz).

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>