staging:iio:adc:ad7150: remove conversion mode handling.

The AD7150 family of devices power up in continues conversion mode.
We can stay in this mode, unless power consumption becomes a real issue.
Actually the event generation as well as the running average
relies on continues conversion mode, so we better stay there.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>