tty/serial: atmel_serial: remove dev_dbg in atmel_set_termios

This fixes 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>