The M4 previously buffered 16K of zeroes for the M0 to transmit, whilst
waiting for the first USB bulk transfer from the host to complete. The
first bulk transfer was placed in the second 16K buffer.
This avoided the M0 transmitting uninitialised data, but was not a
reliable solution, and delayed the transmission of the first
host-supplied samples.
Now that the M0 is placed in TX_START mode, this trick is no longer
necessary, because the M0 can automatically send zeroes until the first
bulk transfer is completed.
As such, the first bulk transfer now goes to the first 16K buffer.
Once the M4 byte count is increased by the bulk transfer completion
callback, the M0 will start transmitting the samples immediately.
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