[S390] hvc_iucv: allocate memory buffers for IUCV in zone DMA

The device driver must allocate memory for IUCV buffers with GFP_DMA,
because IUCV cannot address memory above 2GB (31bit addresses only).

Because the IUCV ignores the higher bits of the address, sending and
receiving IUCV data with this driver might cause memory corruptions.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>