usb: phy: return -ENXIO when PHY layer isn't enabled

in cases where PHY layer isn't enabled, we want
to still return an error code (actually an error
pointer) so that our users don't need to cope with
either error pointer of NULL.

This will simplify users as below:

-	return IS_ERR(phy) ? PTR_ERR(phy) : -ENODEV;
+	return PTR_ERR(phy);

Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Reported-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>