coresight: etm4x: Prohibit TRCSEQSTR modifications while tracing enabled

TRCSEQSTR can be changed by the ETM while a session is active. Its value
therefore needs to be preserved when a sysfs session is disabled so that
users can read the current sequence state after the session ends.

This means that any changes made to TRCSEQSTR while the sysfs-session is
active would be lost when the session is disabled.

Copy the TRCSEQSTR value when disabling a sysfs session, and prohibit
modifying seq_state while the session is active.

Suggested-by: Mike Leach <mike.leach@arm.com>
Reviewed-by: Mike Leach <mike.leach@arm.com>
Signed-off-by: Yeoreum Yun <yeoreum.yun@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20261003-separate_etm_cfg_v2-v13-1-5d8a29aead95@arm.com

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