usb: ehci-s5p: Use devm for requesting ehci_vbus_gpio

The ehci_vbus_gpio is requested but never freed.  This can cause
problems with deferred probes and would cause problems if
s5p_ehci_remove was ever called.  Use devm to fix this.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Tested-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>