iio: ssp_sensors: don't manually free devm managed resources

The charme of devm_* functions is that you don't need to care about them
in the error path. In this case it is valid to just return NULL which makes
the device fail to probe and then the two gpios and the allocated memory
are freed automatically by the driver core.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>