RDMA/qedr: Use pci_enable_atomic_ops_to_root()

Use PCI core interface pci_enable_atomic_ops_to_root() to enable atomic
capability.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Michal Kalderon <michal.kalderon@cavium.com>
CC: Ram Amrani <Ram.Amrani@cavium.com>
CC: Doug Ledford <dledford@redhat.com>