usb: dwc3: xilinx: Fix error handling when getting USB3 PHY

The code that looked up the USB3 PHY was ignoring all errors other than
EPROBE_DEFER in an attempt to handle the PHY not being present. Fix and
simplify the code by using devm_phy_optional_get and dev_err_probe so
that a missing PHY is not treated as an error and unexpected errors
are handled properly.

Fixes: 84770f02

 ("usb: dwc3: Add driver for Xilinx platforms")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Link: https://lore.kernel.org/r/20220126000253.1586760-3-robert.hancock@calian.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>