[SERIAL] 8250 serial console update uart_8250_port ier

On some embedded PowerPC (MPC834x) systems an extra byte would some
times be required to flush data out of the fifo.
serial8250_console_write() was updating the IER in hardware without
also updating the copy in uart_8250_port. This causes issues functions
like serial8250_start_tx() and __stop_tx() to misbehave.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>