exynos: usb: Fix data abort on boards w/o vbus-gpio node in the DT

Commit 4a271cb1 doesn't take into account that fdtdec_setup_gpio()
returns success when the gpio passed to it is FDT_GPIO_NONE (no
gpio node found in the fdtdec_decode_gpio() call). This results in
calling gpio_direction_output() on invalid gpio. For this reason
executing "usb start" command on Arndale causes data abort in the
ehci-exynos driver.

Add the fdt_gpio_isvalid() check to fix that problem.

Signed-off-by: Andrey Konovalov <andrey.konovalov@linaro.org>
Cc: Julius Werner <jwerner@chromium.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Cc: Marek Vasut <marex@denx.de>