powerpc/usb: Depricate usb_phy_type and usb_dr_mode uboot env variables

Remove getting values of usb mode and phy_type from "usb_dr_mode"
and "usb_phy_type" uboot env variables. Now, these are determined
only from hwconfig string

Signed-off-by: Ramneek Mehresh <ramneek.mehresh@freescale.com>
Acked-by: York Sun <yorksun@freescale.com>