iio: dac: ad5064: Value returned by ad5064_vref_name may not be 'const * const'

Fixes the following W=1 kernel build warning(s):

 drivers/iio/dac/ad5064.c:790:8: warning: type qualifiers ignored on function return type [-Wignored-qualifiers]
 790 | static const char * const ad5064_vref_name(struct ad5064_state *st,
 | ^~~~~

Cc: Michael Hennerich <Michael.Hennerich@analog.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>