rtc: at91sam9: properly handle error case

In case of a probe error, it is possible to abort after issuing
clk_prepare_enable(). Ensure the clock is disabled and unprepared in that
case.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>