coresight: Add helper for setting csdev->mode

coresight: Add helper for setting csdev->mode

Now that mode is in struct coresight_device, sets can be wrapped. This
also allows us to add a sanity check that there have been no concurrent
modifications of mode. Currently all usages of local_set() were inside
the device's spin locks so this new warning shouldn't be triggered.

coresight_take_mode() could maybe have been used in place of adding
the warning, but there may be use cases which set the mode to the same
mode which are valid but would fail in coresight_take_mode() because
it requires the device to only be in the disabled state.

Signed-off-by: James Clark <james.clark@arm.com>
Link: https://lore.kernel.org/r/20240129154050.569566-13-james.clark@arm.com
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>