Fixed problem with tone_hi - tone_lo computation being done
with unsigned arithmetic, and thus always giving a positive result.

I forgot to sync the changes relating to things like 'dac_vol_right(..)'
& 'dac_vol_left(..)' removed in favor of 'dac_vol(left,right)' instead;
applies to 'volume(..)', 'lo_lvl(..)' & 'dac_vol(..)'.

removed 'route(..)' due not easily supportable atm. Updated
'dap_enable(..)' in lieu of route. Fixed dap_audio_eq_band(..) bad use
of unsigned. Changed 'updateCoefs(..)' so default behavior is not reset
other three elements of state.

Something wrong with my attempt to apply the math in the datasheet,
CHIP_LINE_OUT_VALUE is better set to 0x0505 rather than 0x0D0D.

I thought the time taken to read back and then write zero back to the
DAP_FILTER_COEF_ACCESS WR bit would be enough but coefficients did not
load to the selected filter, tried 1ms delay instead still didn't work -
chances are that either not zero-ing or only zero-ing just before
writing the DAP_COEF_WR_xx_xSB registers would be fine but for now I'd
prefer to zero it sooner than later.

load_peq(..); wasn't writing the INDEX and WR bits in
DAP_FILTER_COEF_ACCESS

added AudioBiquadFilter::
public:
void updateCoefs(int *source, bool noReset);
void updateCoefs(int *source);

added AudioControlSGTL5000::
private:
unsigned int modify(unsigned int reg, unsigned int val, unsigned int
iMask);

public:
unsigned int micGain(unsigned int n) { return modify(0x002A, n&3, 3); }
unsigned short lo_lvl_right(uint8_t n);
unsigned short lo_lvl_left(uint8_t n);
unsigned short lo_lvl(uint8_t n);
unsigned short dac_vol_right(float n);
unsigned short dac_vol_left(float n);
unsigned short dac_vol(float n);
unsigned short dap_mix_enable(uint8_t n);
unsigned short dap_enable(uint8_t n);
unsigned short dap_peqs(uint8_t n);
unsigned short dap_audio_eq(uint8_t n);
unsigned short dap_audio_eq_band(uint8_t bandNum, float n);
void dap_audio_eq_geq(float bass, float mid_bass, float midrange, float
mid_treble, float treble);
void dap_audio_eq_tone(float bass, float treble);
void load_peq(uint8_t filterNum, int *filterParameters);
void route(uint8_t via_i2s, uint8_t via_dap);

PlayMidiTones example modified to work with ramping.

using user-supplied coefficients