serial: atmel: trivial: clean the IP version decoding code

No functional change is associated with this patch.

A driver property depends on the Atmel serial IP revision. This property
is the way the rx timeout is handled: by an hardware or software timer.
So, change this property name and setup code so that it's easier to understand
and more future proof as the distinction of USART vs. UART is blurrier on newer
SoCs.
Variable names and debug comments are also adapted to make this code more obvious.

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