iio: dac: vf610: Fix an error handling path in 'vf610_dac_probe()'

A call to 'vf610_dac_exit()' is missing in an error handling path.

Fixes: 1b983bf4

 ("iio: dac: vf610_dac: Add IIO DAC driver for Vybrid SoC")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>