boards: arduino_nano_33_ble: Fix board initialization code

This is a follow-up to commit 1b479d6a.

Use the `GPIO_OUTPUT_HIGH` flag instead of `GPIO_OUTPUT_INIT_HIGH`
as the latter does not actually configure the pin as an output
(it does not include `GPIO_OUTPUT`) what causes an assertion in
`gpio_pin_configure()` to fail.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>