tty/serial: at91: fix uart/usart selection for older products

Since commit 055560b0

 (serial: at91:
distinguish usart and uart) the older products which do not have a
name field in their register map are unable to use their serial output.
As the main console output is usually the serial interface (aka DBGU) it
is pretty unfortunate.
So, instead of failing during probe() we just silently configure the serial
peripheral as an uart. It allows us to use these serial outputs.
The proper solution is proposed in another patch.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>