s390/3270: Fix buffer assignment

Since commit 1b2ac5a6 ("s390/3270: use new address translation
helpers") rq->buffer is passed unconditionally to virt_to_dma32().
The 3270 driver allocates requests without buffer, so the value passed
to virt_to_dma32 might be NULL. Check for NULL before assigning.

Fixes: 1b2ac5a6 ("s390/3270: use new address translation helpers")
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>