usb: xhci: Fix uninitialized variable compiler warning

Fix the following compiler warning:

drivers/usb/host/xhci-hub.c: In function 'xhci_hub_control':
drivers/usb/host/xhci-hub.c:544:6: warning: 'retval' may be used
uninitialized in this function error, forbidden warning: xhci-hub.c:544
make[7]: *** [drivers/usb/host/xhci-hub.o] Error 1

Change-Id: I4f7ab5fcfeaa3de9b9acc48571a92771935bc0be
Signed-off-by: Jack Pham <jackp@codeaurora.org>