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

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

Commit 4a271cb1b4ff 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 <[email protected]>
Cc: Julius Werner <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Minkyu Kang <[email protected]>
Cc: Marek Vasut <[email protected]>