LoongArch: Remove a bogus reference to ZONE_DMA

commit c37325cb upstream.

Remove dead code. LoongArch does not have a DMA memory zone (24bit DMA).
The architecture does not even define MAX_DMA_PFN.

Cc: stable@vger.kernel.org
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Signed-off-by: Petr Tesarik <ptesarik@suse.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>