coresight: Add a helper for getting csdev->mode

Now that mode is in struct coresight_device accesses can be wrapped.

Signed-off-by: James Clark <james.clark@arm.com>
Link: https://lore.kernel.org/r/20240129154050.569566-12-james.clark@arm.com


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