coresight: Fix dsb_mode_store() unsigned val is never less than zero

dsb_mode_store() warn: unsigned 'val' is never less than zero.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202410150702.UaZ7kvet-lkp@intel.com/


Fixes: 018e43ad ("coresight-tpdm: Add node to set dsb programming mode")
Signed-off-by: Pei Xiao <xiaopei01@kylinos.cn>
Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/122503017ada249fbf12be3fa4ee6ccb8f8c78cc.1732156624.git.xiaopei01@kylinos.cn