iio: core: introduce iio_device_{claim|release}_buffer_mode() APIs

These APIs are analogous to iio_device_claim_direct_mode() and
iio_device_release_direct_mode() but, as the name suggests, with the
logic flipped. While this looks odd enough, it will have at least two
users (in following changes) and it will be important to move the IIO
mlock to the private struct.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20221012151620.1725215-2-nuno.sa@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>