Filter Wizard: properly scale RF bandwidth when Hz/kHz/MHz scale changes

Signed-off-by: Tim Harder <timothy.harder@analog.com>
Filter Wizard: scale Fcutoff when frequency units are changed

This also fixes issues when advanced settings are enabled, the frequency
units are changed, and then a filter is designed. Previously the Fcutoff
value wouldn't have been scaled during the frequency unit change so the
process would have failed or generated a bad filter.

Signed-off-by: Tim Harder <timothy.harder@analog.com>