tty/serial: atmel_serial: check state if serial port closed

It is possible that port has been closed before tasklet is executed.
Just return if port is closed.

Signed-off-by: Leilei Zhao <leilei.zhao@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
tty/serial: atmel_serial: remove dev_dbg in atmel_set_termios

This fix a driver bug which stopped the whole system (in case of
serial console).
This log message is not useful anyway as this information
is printed elsewhere.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>