s390/pci_dma: remove memset from dma_alloc

Get rid of a useless memset from dma_alloc. Users of dma_alloc who want
zero initialized memory can get it by specifying __GFP_ZERO or use one
of the zalloc variants.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>