serial: 8250_exar: Clear buffer before shutdown

When closing and shutting down the exar serial port, if the chip
has not finished sending all of the data in its buffer, the
remaining bytes will be lost.  Hold off on the shutdown until the
bytes have all been sent.

Signed-off-by: Robert Middleton <robert.middleton@rm5248.com>
Link: https://lore.kernel.org/r/20190801145640.26080-1-robert.middleton@rm5248.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>