usb: renesas_usbhs: rename phy to usb_phy in usbhs_priv

To support a generic phy driver in this driver later, this patch
renames "struct usb_phy *phy" to "struct usb_phy *usb_phy".

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>