of: make of_node_check_flag() device_node parameter const

The device_node argument isn't modified by of_node_check_flag(), so mark it
const.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Link: https://lore.kernel.org/r/20211014173055.2117872-1-nathanl@linux.ibm.com


Signed-off-by: Rob Herring <robh@kernel.org>