drm/panthor: Fix panthor_devfreq kerneldoc

Missing '*' to have a valid kerneldoc prefix.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202403031019.6jvrOqGT-lkp@intel.com/


Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240304090812.3941084-2-boris.brezillon@collabora.com