IB/qib: Use pci_is_root_bus() to check whether it is a root bus

Use pci_is_root_bus() instead of "if (bus->parent)" statement
for better readability.

Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Mike Marciniszyn <mike.marciniszyn@intel.com>