phy: armada375-usb2: Set drvdata for phy and use it

At the context we have pointer to struct phy, it's useful to call
phy_get_drvdata() to get the address of cluster_phy. This has slightly
better readability than calling dev_get_drvdata(phy->dev.parent).

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>