usb: chipidea: fix no transceiver case

Since usb phy code does return ERR_PTR() values, make sure that we don't
end up dereferencing them. This is a problem, for example, on platforms
that don't register a phy for chipidea since b7fa5c2a ("usb: phy: return
-ENXIO when PHY layer isn't enabled").

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>