cdc_uart: performance improvements

[P33M]

This commit fixes a few deadlock scenarios and makes cdc_task more amenable to being called in a threadable context. Full-duplex is now reliable at fast and slow baudrates.

There is still an annoyance where if the CDC interface is deactivated while the UART RX is still active, buffers queued to the endpoint hardware aren't flushed. This is a tinyUSB interface bug.

Signed-off-by: Jonathan Bell [email protected]