When using half-duplex communication (e.g. only a single wire for both
Tx and Rx) a data packet needs to be transmitted entirely before
attempting to switch into receiving mode.
For legacy serial hardware, the tcdrain() probably works as advertised,
and waits until the data has been transmitted. However for common
usb-serial converters, the hardware doesn't provide any feedback to the
driver, and the tcdrain() function can only wait until the data has been
transmitted to the usb-serial chip. There is no guarantee that the data
has actually been transmitted by the usb-serial chip.
As a workaround, we wait at least the minimum amount of time required to
transmit the data packet over a serial line, taking into account the
current configuration.
