phy: berlin-usb: Set drvdata for phy and use it

At the context where we have pointer to struct phy, it's useful to call
phy_get_drvdata() to get the address of priv. With this change, we can
remove the to_phy_berlin_usb_priv() macro and remove *phy from struct
phy_berlin_usb_priv.

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