iio: core: Fix handling of 'dB'

This patch fixes the call to iio_str_to_fixpoint when using 'dB' sufix.
Before this the scale_db was not used when parsing the string written to
the attribute and it failed with invalid value.

Fixes: b8528224 ("iio: core: Handle 'dB' suffix in core")
Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>