phy: omap-usb2: move omap_usb.h from linux/usb/ to linux/phy/

No functional change. Moved omap_usb.h from linux/usb/ to linux/phy/.
Also removed the unused members of struct omap_usb (after phy-omap-pipe3
started using it's own header file)

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>