iio: pressure: bmp280: Use IS_ERR() in bmp280_common_probe()

iio: pressure: bmp280: Use IS_ERR() in bmp280_common_probe()

`devm_gpiod_get_optional()` may return non-NULL error pointer on failure.
Check its return value using `IS_ERR()` and propagate the error if
necessary.

Fixes: df6e71256c84 ("iio: pressure: bmp280: Explicitly mark GPIO optional")
Signed-off-by: Salah Triki <salah.triki@gmail.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250818092740.545379-2-salah.triki@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>