usb: phy: Fix Kconfig dependency for Phy drivers

USB phy layer driver are only built if usb host is selected, but they
are used too by USB_GADGET drivers

Signed-off-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Acked-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>