net/rocker: Cleanup the useless return value check

None of pci_dma_read()'s callers check the return value except
rocker. There is no need to check it because it always return
0. So the check work is useless. Remove it entirely.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Mao Zhongyi <maozy.fnst@cn.fujitsu.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
