Filter Wizard: initialize PLL div option to show correct value

Previously it would default to show whatever option was set via GUIDE
during the GUI setup.

Signed-off-by: Tim Harder <timothy.harder@analog.com>
Filter Wizard: disable apply button right after it's pressed

Previously no GUI update was forced so it would appear disabled when the
next update occurred.

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