coresight: Add helper for atomically taking the device

Now that mode is in struct coresight_device, this pattern can be wrapped
in a helper.

Signed-off-by: James Clark <james.clark@arm.com>
Link: https://lore.kernel.org/r/20240129154050.569566-11-james.clark@arm.com


Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>