drivers: gpio: npm2100: properly handle I2C write return value

properly check return code from i2c_reg_write_byte_dt effectively
making it possible for gpio_npm2100_port_set_masked_raw to error out since:

if (ret != 0U) {
	return ret;
}

was dead code before this fix.

Fixes CID-444376

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>