clocksource/drivers/timer-atmel-pit: Properly handle error cases

The smatch utility reports a possible leak:

smatch warnings:
drivers/clocksource/timer-atmel-pit.c:183 at91sam926x_pit_dt_init() warn: possible memory leak of 'data'

Ensure data is freed before exiting with an error.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>