USB: cp210x: add tx_empty()

Added tx_empty callback needed for generic wait-until-sent support.
Without this function, when the port is closed usbserial can't know that
there are still data in the chip's transmit FIFO. The chip gets disabled
and untransmitted data lost. When the actual byte count is reported by
tx-empty the close can be delayed until all data are sent.

Signed-off-by: Konstantin Shkolnyy <konstantin.shkolnyy@gmail.com>
[johan: modify tx_empty error handling ]
Signed-off-by: Johan Hovold <johan@kernel.org>