audiofx: Use most common convention for definitions of IIR filter coefficients.

audiofx: Use most common convention for definitions of IIR filter coefficients.

Most signal processing texts, including MATLAB, use the following convention for IIR filter coefficients:

a_0 y[n] + a_1 y[n-1] + ... + a_M y[n-M] = b_0 x[n] + b_1 x[n-1] + ... + b[N] x[n-N]

Usually, a_0 is set to 1 because the coefficients can always be rescaled, giving

y[n] = b_0 x[n] + b_1 x[n-1] + ... + b[N] x[n-N] - a_1 y[n-1] - ... - a_M y[n-M]

The convention that was previously used by audiofxbaseiirfilter and derived class had the a and b coefficients swapped, and did not have the minus signs.

This change makes the audiofx plugin use the more common convention described above.