gpio: of: drop needless gpio_chip look-up in of_parse_own_gpio()

This function is doing more complicated than needed.  The caller of
this function, of_gpiochip_scan_gpios() already knows the pointer to
the gpio_chip.  It can pass it to of_parse_own_gpio() instead of
looking up the gpio_chip by gpiochip_find().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>