The sample frequency is an artificial parameter used for
isochronous out transfers to better match the target
frequency (usually, a sound card).
when hz is set, devusb adjusts the endpoint's maxpkt to get
the requested frequency and a multiple of the samplesize per
packet.
however, when hz is not set, then we should calculate the
frequency from maxpkt, ntds and pollival, so all parameters
will be consistent with each other.
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