iio: adc: ad_sigma_delta: do not use internal iio_dev lock

Drop 'mlock' usage by making use of iio_device_claim_direct_mode().
This change actually makes sure we cannot do a single conversion while
buffering is enable. Note there was a potential race in the previous
code since we were only acquiring the lock after checking if the bus is
enabled.

Fixes: af300848

 ("iio:adc: Add common code for ADI Sigma Delta devices")
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: <Stable@vger.kernel.org> #No rush as race is very old.
Link: https://lore.kernel.org/r/20220920112821.975359-2-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>