s390/pci: fix potential NULL pointer dereference in dma_free_seg_table()

The dereference to 'zdev' should be moved below the NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>