iio: core: Rework claim and release of direct mode to work with sparse.

Initial thought was to do something similar to __cond_lock()

	do_iio_device_claim_direct_mode(iio_dev) ? : ({ __acquire(iio_dev); 0; })
+ Appropriate static inline iio_device_release_direct_mode()

However with that, sparse generates false positives. E.g.

drivers/iio/imu/st_lsm6dsx/st_lsm6dsx_core.c:1811:17: warning: context imbalance in 'st_lsm6dsx_read_raw' - unexpected unlock

So instead, this patch rethinks the return type and makes it more
'conditional lock like' (which is part of what is going on under the hood
anyway) and return a boolean - true for successfully acquired, false for
did not acquire.

To allow a migration path given the rework is now non trivial, take a leaf
out of the naming of the conditional guard we currently have for IIO
device direct mode and drop the _mode postfix from the new functions giving
iio_device_claim_direct() and iio_device_release_direct()

Whilst the kernel supports __cond_acquires() upstream sparse does not
yet do so.  Hence rely on sparse expanding a static inline wrapper
to explicitly see whether __acquire() is called.

Note that even with the solution here, sparse sometimes gives false
positives. However in the few cases seen they were complex code
structures that benefited from simplification anyway.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250209180624.701140-2-jic23@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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