USB: gpio_vbus: put a missing regulator_put() on error

Note that regulator_put() doesn't care about whether ->vbus_draw is
valid or not.

Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi.px@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>