rtc: sun6i: ensure rtc is kfree'd on error

The error return path on clk_data allocation failure does not kfree
the allocated rtc object. Fix this with a kfree of rtc on the error
exit path.

Detected by CoverityScan, CID#1452264 ("Resource Leak")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>