Ignore the emulated wiimotes speaker data if the sample rate is set at 0hz

Fixes issue 7806.
How about:
if (m_reg_speaker.sample_rate)
        unsigned int sample_rate = sample_rate_dividend / Common::swap16(m_reg_speaker.sample_rate);
else
        unsigned int sample_rate = 0;

Seems a bit more logical and easier to read IMO, but that't just me.