Coresight: Set correct cs_mode for dummy source to fix disable issue

The coresight_disable_source_sysfs function should verify the
mode of the coresight device before disabling the source.
However, the mode for the dummy source device is always set to
CS_MODE_DISABLED, resulting in the check consistently failing.
As a result, dummy source cannot be properly disabled.

Configure CS_MODE_SYSFS/CS_MODE_PERF during the enablement.
Configure CS_MODE_DISABLED during the disablement.

Fixes: 9d3ba0b6 ("Coresight: Add coresight dummy driver")
Signed-off-by: Jie Gan <quic_jiegan@quicinc.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20240812042844.2890115-1-quic_jiegan@quicinc.com