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

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>