sparc32: dma_alloc_coherent must honour gfp flags

dma_zalloc_coherent() calls dma_alloc_coherent(__GFP_ZERO)
but the sparc32 implementations sbus_alloc_coherent() and
pci32_alloc_coherent() doesn't take the gfp flags into
account.

Tested on the SPARC32/LEON GRETH Ethernet driver which fails
due to dma_alloc_coherent(__GFP_ZERO) returns non zeroed
pages.

Signed-off-by: Daniel Hellstrom <daniel@gaisler.com>
Signed-off-by: David S. Miller <davem@davemloft.net>