[sma_corr] Added a reset to the xn_num register (reset with xacc_rst signal).

This prevents the xengine accumulator from getting locked up forever at start up.
It will still get locked up but can now be brought out of lock up with a reset.
[sma_corr] Lowered the xn_num initial value from 30 to 8.

This should mean a shorter wait for the first accumulation on startup.
[sma_corr] Fixed incorrect casting of xn_num register.

This made it seem like the xn_num logic was zero and thus necessitating the
previous few commits however it was just being casted incorrectly.
[sma_corr] Added counters for the FFT overflow indicators.
[cgain_core] Removed the 12 fractional bits of cgain/gain signal.

This is hardly ever used, i.e. usually we need HIGHER levels going into
the quantizers, and actually is limiting the amount of shifting we can do
in the FFT. Now with a full 16 bits of gain we can handle shifting at every
stage which should improve the handling of strong tones in the FFT.