iio: dac: ad5592r: Delete stray unlock in ad5592r_write_raw()

This code was converted to use guard locks but one of the unlocks was
accidentally overlooked.  Delete it.

Fixes: 135e101f ("iio: dac: ad5592r: use lock guards")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://patch.msgid.link/Z_-P7bsD3KL5K25R@stanley.mountain


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>