Fix hardware mistakes

This commit adds fixes some of the mistakes identified after the
board was ordered.

Add 100n capacitor to MCUs reset pin to fix programming issues.

Add series connection of resistor and capacitor in parallel
with mixer output amplifier to fix the DC balance issue.

Remove the now unneeded capacitor and resistor
after the low-pass filter.

Add 200k resistor in feedback path of single-ended to differential
conversion circuit to solve the issue with limited input voltage range.