sam3u2c - return real free write size in uart buf
sam3u2c - return real free write size in uart buf

Update the hal implementation of the sam3u2c to return the actual
number of bytes free in the CDC buffer. Previous behavior returned
that there was no space in the buffer if there were any bytes in it.

google/DAPLink-port,sg-/DAPLink,google/DAPLink-port,sg-/DAPLink,google/DAPLink-port,google/DAPLink-port,sg-/DAPLink