Following on from ba940250a5b630018c8d9b0e21c5ed858a20450f, the change here
makes output about 15 times faster (now up to about 550 kbytes/sec).
tinyusb_cdcacm_write_queue will return the number of bytes written, so
there's no need to use tud_cdc_n_write_available.
Signed-off-by: Damien George <damien@micropython.org>
1e4849557d