PCI: imx6: Remove redundant of_node pointer

"np" and "node" are redundant copies of the of_node pointer.  Remove "np"
and use "node" instead.  Replace the "fsl,max-link-speed" use with "node"
as well.  No functional change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>