accel/habanalabs: make gaudi2_is_device_idle() static

This function is only called inside gaudi2.c file.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202303071320.X5ouBlNY-lkp@intel.com/


Signed-off-by: Oded Gabbay <ogabbay@kernel.org>
Reviewed-by: Ofir Bitton <obitton@habana.ai>