imbalance of gain

[qrp73]

There is a little imbalance of gain, which leads to some discontinuities. You can find it by reset calibration and setup DISPLAY TRACE S11 LOGMAG scale=1 with reference position about 27.

@hugen79 tried to fix it in NanoVNA-H, but not complete.

More precise fix is done in this commit: https://github.com/qrp73/NanoVNA/commit/675b5bb3bceeb0b8ee116a1123469d1aebaf5f56