usb: host: ehci-xilinx: adding description for return value

This patch resolves kernel-doc warnings to add return value description
in function comments.

Addressed warnings:

drivers/usb/host/ehci-xilinx-of.c:37: warning: No description found for return value of 'ehci_xilinx_port_handed_over'
drivers/usb/host/ehci-xilinx-of.c:117: warning: No description found for return value of 'ehci_hcd_xilinx_of_probe'
drivers/usb/host/ehci-xilinx-of.c:201: warning: No description found for return value of 'ehci_hcd_xilinx_of_remove'

Signed-off-by: Piyush Mehta <piyush.mehta@xilinx.com>
Link: https://lore.kernel.org/r/20220509170252.28271-1-piyush.mehta@xilinx.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/ehci-xilinx-of.c

@@ -32,6 +32,8 @@
  * There are cases when the host controller fails to enable the port due to,
  * for example, insufficient power that can be supplied to the device from
  * the USB bus. In those cases, the messages printed here are not helpful.
+ *
+ * Return: Always return 0
  */
 static int ehci_xilinx_port_handed_over(struct usb_hcd *hcd, int portnum)
 {
@@ -110,6 +112,8 @@ static const struct hc_driver ehci_xilinx_of_hc_driver = {
  * host controller. Because the Xilinx USB host controller can be configured
  * as HS only or HS/FS only, it checks the configuration in the device tree
  * entry, and sets an appropriate value for hcd->has_tt.
+ *
+ * Return: zero on success, negative error code otherwise
  */
 static int ehci_hcd_xilinx_of_probe(struct platform_device *op)
 {
@@ -194,6 +198,8 @@ static int ehci_hcd_xilinx_of_probe(struct platform_device *op)
  *
  * Remove the hcd structure, and release resources that has been requested
  * during probe.
+ *
+ * Return: Always return 0
  */
 static int ehci_hcd_xilinx_of_remove(struct platform_device *op)
 {