s390/ism: don't pass bogus GFP_ flags to dma_alloc_coherent

dma_alloc_coherent is an opaque allocator that only uses the GFP_ flags
for allocation context control.  Don't pass __GFP_COMP which makes no
sense for an allocation that can't in any way be converted to a page
pointer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Wenjia Zhang <wenjia@linux.ibm.com>