libgomp/plugin/plugin-nvptx.c: Change false to NULL to fix C23 wrong-return-type error [PR117626]

libgomp/ChangeLog:

	PR libgomp/117626
	* plugin/plugin-nvptx.c (nvptx_open_device): Use 'CUDA_CALL_ERET'
	with 'NULL' as error return instead of 'CUDA_CALL' that returns false.