ath79: fix I2C on GL-AR300M devices

ath79: fix I2C on GL-AR300M devices

On GL-AR300M Series GPIO17 described as I2C SDA in Device Tree.
Because of GPIO_OUT_FUNCTION4 register was not initialized on start,
GPIO17 was uncontrollable, it always in high state. According to QCA9531
documentation, default setting of GPIO17 is SYS_RST_L. In order to make
GPIO17 controllable, it should write value 0x00 on bits [15:8] of
GPIO_OUT_FUNCTION4 register, located at 0x1804003C address.

Signed-off-by: Ptilopsis Leucotis <PtilopsisLeucotis@yandex.com>
(cherry picked from commit 57efdd6a2d815d2491c5b7f22ffaeb6a845bfd0a)