Allow optional acap property - sampling_rate

[shantur]

This is needed when using audio capture from a device other than tc358743 such as a UAC2 Capture device to have two way audio over USB

When sampling_rate is provided, janus plugin don't try to query tc358743

[shantur]

With this change, one can create UAC2 capture device for audio input rather than using HDMI->CSI2 -> I2S audio

Sample configs that I tested with

/etc/kvmd/janus/janus.plugin.ustreamer.jcfg

video: {
	sink = "kvmd::ustreamer::h264"
}
acap: {
	device = "plughw:UAC2Gadget,0"
	tc358743 = "/dev/kvmd-video"
    sampling_rate = 48000
}
aplay: {
	device = "plughw:UAC2Gadget,0"
	check = "/run/kvmd/otg/uac2.usb0@meta.json"
}

Replace - https://github.com/pikvm/kvmd/blob/master/kvmd/apps/otg/__init__.py#L119

with

        _write(join(func_path, "c_chmask"), 0b11)
        _write(join(func_path, "c_srate"), 48000)
        _write(join(func_path, "c_ssize"), 2)