ARM: dts: stm32: Fix AV96 WLAN regulator gpio property

The WLAN regulator uses 'gpios' property instead of 'gpio' to specify
regulator enable GPIO. While the former is also currently handled by
the Linux kernel regulator-fixed driver, the later is the correct one
per DT bindings. Update the DT to use the later.

Fixes: 7dd5cbba ("ARM: dts: stm32: Enable WiFi on AV96")
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>