phy: lantiq: rcu-usb2: wait after clock enable

Commit 65dc2e72 ("usb: dwc2: Update Core Reset programming flow.")
revealed that the phy isn't ready immediately after enabling it's
clocks. The dwc2_check_core_version() fails and the dwc2 usb driver
errors out.

Add a short delay to let the phy get up and running. There isn't any
documentation how much time is required, the value was chosen based on
tests.
Signed-off-by: Mathias Kresin <dev@kresin.me>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Acked-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Cc: <stable@vger.kernel.org> # v5.7+
Link: https://lore.kernel.org/r/20210107224901.2102479-1-dev@kresin.meSigned-off-by: Vinod Koul <vkoul@kernel.org>

drivers/phy/lantiq/phy-lantiq-rcu-usb2.c

@@ -124,8 +124,16 @@ static int ltq_rcu_usb2_phy_power_on(struct phy *phy)
 	reset_control_deassert(priv->phy_reset);
 
 	ret = clk_prepare_enable(priv->phy_gate_clk);
-	if (ret)
+	if (ret) {
 		dev_err(dev, "failed to enable PHY gate\n");
+		return ret;
+	}
+
+	/*
+	 * at least the xrx200 usb2 phy requires some extra time to be
+	 * operational after enabling the clock
+	 */
+	usleep_range(100, 200);
 
 	return ret;
 }