usb: phy: fix driver dependencies

both isp1301-omap and fsl_usb2_otg drivers
depend on usb_bus_start_enum() which is only
defined if CONFIG_USB != n. There is a problem,
however, where both those drivers could be
statically linked, while CONFIG_USB=m.

Fix the problem by fixing driver dependency.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>