This was introduced by 35fb90bd57e1a3259aaf67cede50628da6888485, but
it is much simpler and essentially the same to just use
`tud_cdc_n_connected()`.
The only difference is that tud_cdc_n_connected() only checks for DTR,
but this is correct anyway: DTR indicates device presence, RTS indicates
that the host wants to receive data.
Signed-off-by: Damien Tournoud <damien@platform.sh>
fd1e66edb3