usb: gadget: udc: renesas_usb3: should fail if devm_phy_get() returns error

This patch fixes an issue that this driver ignores errors other than
the non-existence of the device, f.e. a memory allocation failure
in devm_phy_get(). So, this patch replaces devm_phy_get() with
devm_phy_optional_get().

Reported-by: Simon Horman <horms+renesas@verge.net.au>
Fixes: 279d4bc6 ("usb: gadget: udc: renesas_usb3: add support for generic phy")
Cc: <stable@vger.kernel.org> # v4.15+
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>