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

There are two error return paths that do not kfree clk_data and
we end up with a memory leak. Fix these with a kfree error exit
path.

Detected by CoverityScan, CID#1402959 ("Resource Leak")

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