Filter Wizard: check the right fields for rx/tx taps

This fixes enabling the save to target button when both filters are
designed before connecting to a target board.

Signed-off-by: Tim Harder <timothy.harder@analog.com>
Filter Wizard: drop old handles.taps usage

Now we split up Rx and Tx taps into different variables (rfirtaps and
tfirtaps) in order to better handle the control flow in the GUI when
designing filters for both channels.

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